to spy out: to scout a land, city, or people secretly in order to gather military or strategic information
TranslatabilityApproximateCertaintySettled
The word pictures someone going out on foot to look a place over before an army moves. It is the standard term for reconnaissance in the conquest stories:
Moses sends men to scout Jazer
Joshua sends two men into Jericho
the tribe of Dan scouts out territory it means to seize
The report a scout brings can be trusted or fatal. Caleb looks back on his own mission with pride, while the men who scout Ai come home with confident advice that leads to a rout. The same act can also be an accusation: Joseph brands his brothers as spies, and David's grieving envoys are charged with casing the very city they came to comfort.
In one case, Absalom's agents fanned out across the tribes, the men do less spying than message carrying, and the word begins to shade toward simply those who go about. That drift is the bridge to the word's other use, for the person who goes about carrying gossip.
to go about spreading damaging reports about someone; to slander
TranslatabilityApproximateCertaintyStrong
The same verb that sends a scout across a border can send a rumor across a community. Here it names the person who goes around carrying damaging tales about a neighbor. The Hebrew picture is vivid: the tongue becomes a road on which reports travel abroad.
This use appears only twice, and both places are stable in the text. The person welcomed into God's presence is the one who has not gone about on his tongue against others, and Mephibosheth complains to King David that a servant carried lies about him.
2Sa 19:27(28)Psa 15:3*
Psa 15:3
Whether the verb here means to slander (go about with tales on the tongue) or, on a minority reading, to spy out with the tongue, which would collapse this use back into the reconnaissance sense.
majorityTo slander: the tongue is pictured as a walkway on which damaging reports travel abroad.; Hans-Joachim Kraus, Psalms 1-59 (Continental Commentary); Frank-Lothar Hossfeld and Erich Zenger, Psalms (Hermeneia)
minorityTo spy out with the tongue, treating the word as its ordinary reconnaissance sense applied rhetorically to speech.; Charles A. Briggs, A Critical and Exegetical Commentary on the Book of Psalms (International Critical Commentary); Mitchell Dahood, Psalms I (Anchor Bible)
Grows from the word for foot, so it carries the sense of going about on foot to look a place over, and it is often a neutral, even praised, reconnaissance rather than an act of betrayal.Jos 14:7Deu 1:24
Both
Both name observing a place secretly to gather information ahead of a decision or an attack.Gen 42:9Jos 2:1
Only spy out
The English word "spy" suggests a covert enemy agent and an air of treachery that the Hebrew does not always carry.
The Hebrew pictures going about on foot carrying tales, which ties slander to the very word used for spying.Psa 15:32Sa 19:27
Both
Both describe spreading damaging reports about a person.Psa 15:3
Only slander
English "slander" is a flat legal-sounding term with no image of movement, losing the picture of a report traveling abroad.
How ragal differs from similar Hebrew words
תּוּרThe word used for the mission in Numbers 13; it means to explore or seek out more broadly, while this word is the narrower term for military spying.
Covert espionage: slipping secret agents into a city or land, often under suspicion of hostile or deceptive intent, so it is the word for accusing someone of being a spy.Gen 42:9Jos 2:12Sa 10:3
Going about on foot to spread slander, the tongue that carries gossip from house to house rather than any scouting of terrain.Psa 15:3
Both
Both send people ahead to reconnoiter foreign territory before an Israelite advance, scouting the land and its cities in preparation for conquest.Jos 7:2Deu 1:24Num 13:2Num 13:17
Searching out and sizing up a region for its quality, its produce, and its people, an assessment of the land itself and not merely a stealthy peek.Num 13:32Num 14:6
Investigating a matter with the mind, seeking things out by wisdom, an intellectual quest with no military edge at all.Ecc 1:13Ecc 2:3Ecc 7:25
Seeking out a place to stop or feed: scouting ahead for a resting spot to camp, or an animal ranging the hills for pasture.Num 10:33Deu 1:33Job 39:8
רָכִילA talebearer or slanderer, close in social meaning to this word's slander sense but built from a different origin, about going about as a trader or peddler.
to scout or reconnoiter, going through territory to gather intelligence, especially of enemy land before an attack.Gen 42:9Jos 2:1Deu 1:24
Both
both grow from the picture of going about among people carrying reports, and the verb can shade into slanderous going-about with the tongue.Psa 15:3Pro 20:19
one who goes about carrying malicious reports about people; the gossip who betrays a confidence.Lev 19:16Pro 11:13Jer 6:28
חָקַרTo search out or investigate; it is paired with this word in accusations of spying, but on its own it means careful inquiry rather than reconnaissance.
to move through a place on foot as a scout, spying out enemy land or a city before an attackGen 42:9Jos 2:1Deu 1:24
to go about carrying tales, using the tongue to slander a personPsa 15:32Sa 19:27
Both
both are used for military reconnaissance, examining a land or city to size it up for attack, and they even appear together in such scenesJdg 18:22Sa 10:3
to probe a matter with the mind, examining and investigating to get to the truth, including God searching the hidden depths of the heartDeu 13:14Psa 44:21Jer 17:10Pro 25:2
ragal in the other biblical languages
κατασκοπέωGreek“to spy”The Greek verb to spy out, used in Galatians of those who spied out the freedom of believers; it matches this word's reconnaissance sense.
Purely a sin of speech: to talk against someone behind their back, running them down with words.Jas 4:111Pe 2:12
What scholars say about ragal
The reference lexica (BDB, HALOT, DCH, Gesenius 18. Auflage, TDOT [Stendebach]) uniformly treat רגל as a single denominative verb from רֶגֶל ("foot"), whose concrete idea "to go about on foot" throws out two specialized branches: the Piel "to scout, reconnoiter" that dominates the conquest narratives, and a rare "to go about with tales, slander" at Psalm 15:3 and 2 Samuel 19:28 (MT). Not one major lexicon posits a separate homonym for the slander sense; 2 Samuel 15:10, where the participle drifts toward couriers who go about, is read as the semantic hinge. Milgrom (JPS Numbers), Weinfeld, and Tigay press a distinction between תור (broad scouting, the term of Numbers 13) and רגל (narrower military espionage and characteristically Deuteronomic diction), while Levine and Budd are more cautious about loading that contrast. On the underlying lexicography the consensus is firm, and James Barr's caution against cognate-driven semantic splitting is the standard reason no shadow root II רגל has been proposed.
In-depth research
This is a research paper, so it may be hard to read without scholarly background. It is useful if you want to dig even deeper into this word.
Sources
BDB, s.v. רגלlexicon
HALOT, s.v. רגלlexicon
DCH, s.v. רגלlexicon
F. J. Stendebach, "רֶגֶל," Theological Dictionary of the Old Testament, vol. XIIIdictionary
Jacob Milgrom, Numbers (JPS Torah Commentary), Excursus on chapter 13commentary