before vowels mult-, word-forming element meaning "many, many times, much," from combining form of Latin multus "much, many," from PIE *ml-to-, from root *mel- (2) "strong, great, numerous." It was much-used in forming Latin compounds in classical times and after (such as multianimis "having much courage," multibibus "much-drinking," multicomus "having much hair," multiloquus "talkative"). Many English words that use it (multinational, etc.) are 20c. coinages.

Proto-Indo-European root meaning "strong, great." It forms all or part of: ameliorate; amelioration; meliorate; melioration; meliorism; multi-; multiform; multiple; multiply; multitude. It is the hypothetical source of/evidence for its existence is provided by: Greek mala "very, very much;" Latin multus "much, many," melior "better."  

c. 1600, "of vast extent;" 1620s, "consisting of a great number," from Latin multitudin-, stem of multitudo (see multitude) + -ous. First in Shakespeare or Dekker, depending on the dating of their publications, though it is certainly "Macbeth" that has fixed it in the language. Related: Multitudinously; multitudinousness.

Will all great Neptune's ocean wash this blood

Clean from my hand? No; this my hand will rather

The multitudinous seas incarnardine,

Making the green one, red.