really: as a matter of actual fact, so that a thing is what it is called and not merely called it
This word is built straight off the verb "to be," and that is what it does: it says that something actually is the case. When a speaker adds it, a claim stops being merely reported and is vouched for as fact. The officer at the cross in Luke 23:47 is not offering an opinion about the man he has just executed; he is stating what he now takes to be so.
In most of its uses it strengthens a whole statement, and "really," "truly," or "indeed" fits comfortably. The letters to Timothy use it a second way, setting it in front of a noun so that it sorts the true case from the merely nominal one: the widows who are actually alone and dependent, as against every woman the label could technically cover, and "the life that is really life" as against the comfortable existence money can buy.
Greek had other ways of saying "truly." The adverb ἀληθῶς, also rendered "truly," works from the side of truth, marking a report as accurate rather than false, and the adjective ἀληθινός, usually rendered "true," marks a thing as the real article rather than a copy. This word works from the side of existence: whatever is in question actually is.