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."