You use least to indicate that something is true or happens to a smaller degree or extent than anything else or at any other time. He had a way of throwing her off guard with his charm when she least expected it.
Definitions of least adjective 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” see more adverb
There are 12 meanings listed in OED's entry for the word least, one of which is labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.
