"having many names or terms," 1610s, from Late Latin multinominis "many-named," from multi- "many" (see multi-) + Latin nomen (genitive nominis) "name," cognate with Old English nama (from PIE root *no-men- "name").
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.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nama; Avestan nama; Greek onoma, onyma; Latin nomen; Old Church Slavonic ime, genitive imene; Russian imya; Old Irish ainm; Old Welsh anu "name;" Old English nama, noma, Old High German namo, Old Norse nafn, Gothic namo "name."
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Harper, D. (n.d.). Etymology of multinominous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/multinominous