Old English mast "greatest in number, amount, or extent; largest," earlier mæst, from Proto-Germanic *maistaz (source also of Old Saxon mest, Old Frisian mast, Old Norse mestr, Dutch meest, German meist, Gothic maists "most"), superlative form of Proto-Germanic *maiz, root of Old English ma, mara (see more). Used in Old English as superlative of micel "great, large" (see mickle), hence, in later use, superlative of much. The vowel has been influenced by more.

Original sense of "greatest" survives in phrase for the most part (mid-14c.; late Old English had þa mæste dæl). Slang the most meaning "the best, extremely good" is attested from 1953. Also used as an adverb in Old English and in late Old English as a noun, "the greatest or greater number." The sense of "greatest value or advantage" in the phrase make the most of (something) is by 1520s. Related: Mostly.

Double superlative mostest "greatest amount or degree" is by 1849 in U.S. Southern and African-American vernacular. The formula for victory in battle attributed to famously unschooled Confederate Lt. Gen. Nathan Bedford Forrest is first attested (1886) as Git thar the fastest with the mostest men.

From 15c.-17c. English also had mostwhat "for the most part," mostwhen "on most occasions," mostwhere "in most places."

"best," 1743, jocular, from better (adj.) + -most.

also bottommost, "situated at the very lowest," 1861, from bottom (adj.) + -most.

1640s, from eastern + -most. Eastermost attested from 1610s; comparative eastermore from late 15c.

1725, from end (n.) + -most. Middle English had endemest (adv.) "from end to end, throughout."

Middle English formest, from Old English fyrmest, formest "earliest, first, most prominent," from Proto-Germanic *furmista-/*frumista- (related to Old English fruma "beginning"), from PIE *pre-mo-, suffixed (superlative) form of root *per- (1) "forward," hence "through, in front of, before, first" + additional superlative suffix -est. For the -m-, see -most, and compare similarly formed Old Frisian formest, Gothic frumists. Altered on the assumption that it is a compound of fore and most. The same compound without the superlative -m- is first. Also in Old English as an adverb, "first of all, at first, in the first place."

late 14c., hyndermest; see hinder (adj.) + -most. Middle English had also hindermore, which, as a noun, could mean "the hinder parts."

"furthest at the rear," late 14c., from hind (adj.) + -most.

Thra. What, if a toy take 'em i' the heels now, and they run all away, and cry, 'The devil take the hindmost'?

Dion. Then the same devil take the foremost too, and souse him for his breakfast! [Beaumont & Fletcher, "Philaster," Act V, Scene 2, 1611]

16c. respelling of Middle English innemest, from Old English innemest "furthest within, remotest from the boundary;" see in + -most.

mid-14c., from inner + -most. In the same sense innerest is from c. 1200. The older word is inmost. Innermore also existed in Middle English.