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."