Webster's Revised Unabridged Dictionary · 1913

Least

Least (lēst) , adjective

[Old English last, lest, Anglo-Saxon lasast, lasest, superl. of lassa less. See Less, a.]

Smallest, either in size or degree; shortest; lowest; most unimportant; as, the least insect; the least mercy; the least space. [Used as the superlative of little.]
I am the least of the apostles. — 1 Cor. xv. 9
He who tempts, though in vain, at least asperses The tempted with dishonor. — Milton
Upon the mast they saw a young man, at least if he were a man, who sat as on horseback. — Sir P. Sidney

Least is often used with the, as if a noun.

Collocations (1)
At least or At the least , at the least estimate, consideration, chance, etc.; being no less than; hence, at any rate; at all events; even. See However.

It takes as its fundamental principle that the most probable values are those which make the sum of the squares of the residual errors of the observation a minimum.

Least , adverb

In the smallest or lowest degree; in a degree below all others; as, to reward those who least deserve it.

Least , conjunction

See Lest, conj. [Obsolete] — Spenser