lower, lowest: placed underneath, from the lower of two things down to the deepest place there is.
TranslatabilityApproximateCertaintyStrong
This is the adjective built from the everyday word for "under" or "beneath" (תַּחַת). It describes something as the lower one: the lower of two springs, the bottom deck of the ark, the foot of a mountain. In these plain uses it simply marks position, the opposite of "upper" (עִלִּי), which stands against it in the story of the upper and lower springs.
Stretched to its extreme, the same word names the very bottom of everything. In poetry "the lowest parts of the earth" and "the lowest pit" reach down past any ordinary basement to the depths of the grave and the world of the dead (שְׁאוֹל). To sink to the lowest place is to die or to be as good as dead, and to be lifted out of it is rescue from death itself.
So one adjective runs the whole vertical range. On the surface it points to the lower of two nearby things; at its limit it points to the deepest, darkest place a person can go.
Nuances
1athe lower of two positions: the one placed beneath its counterpart.
TranslatabilityDirect
Gen 6:16Exo 19:17Jos 15:19Jdg 1:15Neh 4:13(7)Job 41:24(16)
1bthe lowest depths: the deepest parts of the earth, the pit, and the realm of the dead.
It reaches past "lower" to "lowest", naming the very bottom rather than the lower of two. God's fire burns down to the lowest reach of the grave, a sufferer is set in the lowest pit and calls out of it, and Nehemiah stations the people at the lowest open spots behind the wall.Deu 32:22Psa 86:13Psa 88:6Lam 3:55Neh 4:13
Only this form names the depths of the earth as a region in its own right, the land below where the dead are laid. The psalms put the unborn and the doomed there, Isaiah calls on it to shout for joy, and Ezekiel makes it the destination of fallen empires who go down to the pit.Psa 63:9Psa 139:15Isa 44:23Ezk 26:20Ezk 31:14Ezk 32:24
It also marks the plain physical bottom of things outside any building plan: the foot of the mountain where Israel took its stand, the lower springs handed over with a stretch of dry land, and the lower stone of a grinding pair, whose hardness Job borrows to picture an unmoved heart.Exo 19:17Jos 15:19Jdg 1:15Job 41:24
Both
Both can label the bottom storey of something built in stacked levels, with a middle and an upper level named right alongside it: the ark's three decks, the side chambers of Solomon's temple, the tiered rooms of Ezekiel's.1Ki 6:6Ezk 41:7Ezk 42:5Gen 6:16
Both work by contrast rather than by fixed height. Each is normally spoken in the same breath as its upper counterpart, so a reader learns only that this one sits under that one: the lower town and the upper town, the lower springs and the upper springs.1Ch 7:242Ch 8:5Jos 15:19Jdg 1:15
This is the form that hardens into a place name. The border town on the Judah and Ephraim line is simply Lower Beth Horon, the standing partner of an Upper Beth Horon on the ridge above it, and the label follows the town through boundary lists, building notices, and family records alike.Jos 16:3Jos 18:131Ki 9:171Ch 7:242Ch 8:5
It is the word of the measured building plan, tagging which of a structure's named parts is the lower one: the lower gate, the lower pavement, the lower ledge of the altar, the lower reservoir in Jerusalem. The interest is architectural and precise, part of a description that also gives dimensions.Ezk 40:18Ezk 40:19Ezk 43:14Isa 22:9
taḥti in the other biblical languages
κατώτεροςGreek“lower”the Greek "lower," used in "the lower parts of the earth" (Ephesians 4:9), which echoes this Hebrew phrase for the depths.
What scholars say about taḥti
The adjective תַּחְתִּי is a straightforward derivative of the preposition תַּחַת "under, beneath," and the lexica (BDB, HALOT, DCH) agree on its sense: "lower, lowest, nether." Its concrete uses are uncontroversial (the lower springs of Josh 15:19 // Judg 1:15; the lowest deck of the ark in Gen 6:16; the lower millstone in Job 41:24; the lower slope of Sinai in Exod 19:17). The interpretive discussion concerns the stock phrases אֶרֶץ תַּחְתִּיּוֹת / תַּחְתִּיּוֹת אֶרֶץ ("the lowest parts of the earth") and בּוֹר תַּחְתִּיּוֹת / שְׁאוֹל תַּחְתִּית ("the lowest pit" / "the lowest Sheol"). Commentators and the theological dictionaries debate whether these denote the underworld proper as a distinct netherworld, or function more generally as poetic superlatives for "the depths" and, by extension, death and mortal danger; the superlative force of תַּחְתִּי with שְׁאוֹל is often taken as intensification ("the very depths of Sheol") rather than as evidence for tiered cosmology. Psalm 139:15 is a special case, where "the lowest parts of the earth" is widely read as a poetic figure for the concealed depths of the womb. These are questions about the referent of the phrases, not about the meaning of the adjective, which is secure.