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.

before vowels myc-, word-forming element meaning "mushroom, fungus," formed irregularly from Latinized form of Greek mykēs "fungus, mushroom, anything shaped like a mushroom," a word of uncertain origin (Beekes doubts the traditional explanation that connects it to the source of mucus). The correct form is myceto- (mycet-).

before vowels myel-, word-forming element meaning "marrow, spinal cord," from Greek myelos "marrow; the brain," a word of unknown origin.

commercial word-forming element denoting devices that work automatically or businesses containing self-service equipment, abstracted from automat (1903), which probably is from automatic, in which case the element is etymologically from Greek matos "thinking, animated."

comparative word-forming element added to already comparative adjectives and adverbs, Middle English (innermore, outermore, furthermore, overmore, etc.), from more (adv.). The formation also was in Old Norse and the English use might be from Scandinavian.

word-forming element in science, used to form names of antibiotic compounds derived from fungi, from Latinized form of Greek mykēs "fungus, mushroom" (see myco-).