least (adj.)

Old English læst, earlier læsest "smallest, lowest in power or position" (superlative of little (adj.)), from Proto-Germanic superlative *laisista-, from PIE root *leis- (2) "small" (see less). Qualifying phrase at least "not to say more than is certainly true" is Middle English æt læstan, from the notion of "at the lowest degree." As a noun, "smallest admissible quantity or degree," from early 12c.; as an adverb, Old English læst "in the least degree."

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Definitions of least from WordNet
least (n.)
something that is of no importance;
that is the least of my concerns
it is the least I can do
least (adv.)
used to form the superlative;
The garter snake is the least dangerous snake
Synonyms: to the lowest degree
least (adj.)
the superlative of `little' that can be used with mass nouns and is usually preceded by `the'; a quantifier meaning smallest in amount or extent or degree;
didn't care the least bit
he has the least talent of anyone