Old English mast "greatest number, amount, extent," earlier mæst, from Proto-Germanic *maistaz (cf. 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). Vowel influenced by more. Original sense of "greatest" survives in phrase for the most part (c.1400). Slang meaning "the best, extremely good" is attested from 1953. Also used as an adverb in Old English. Phrase make the most of (something) is by 1520s. Related: Mostly. Double superlative mostest is 1885, from U.S. Southern and Black English.
Old English monig, manig "many, many a, much," from Proto-Germanic *managaz (cf. Old Saxon manag, Swedish mången, Old Frisian manich, Dutch menig, Old High German manag, German manch, Gothic manags), from PIE *menegh- "copious" (cf. Old Church Slavonic munogu "much, many," Old Irish menicc, Welsh mynych "frequent," Old Irish magham "gift"). Pronunciation altered by influence of any (see manifold).
Old English menigu, from many (adj.). The many "the multitude" attested from 1520s. Cf. also Gothic managei "multitude, crowd," Old High German managi "large number, plurality," German Menge "multitude."