least: the smallest or lowest of all, whether in size, importance, or standing
This is the superlative of the ordinary word for "small," μικρός. Where that word means simply "little," this one means the smallest or lowest of all: the very least. Greek builds its "less / least" pair from a separate root, so the comparative partner is ἐλάσσων, "less" or "smaller," and this word caps it off as "least."
The scale it measures shifts with the thing it describes. It can rank size or amount (the tiny rudder that steers a ship, being faithful with a very small trust), the importance of a thing (the least of the commandments, a trivial court case), or the standing of a person (the least in the kingdom, the least of the apostles). The core idea stays fixed: bottom of the scale.
The word often lands with weight. Paul stacks a further comparative on top of the superlative to call himself "less than the least of all the saints," Bethlehem is told it is "by no means least" among Judah's towns, and Jesus ties faithfulness "in the least" to being entrusted with much.