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word-forming element meaning "under," from Latin preposition sub "under" (also "close to, up to, towards"), from a variant form (*(s)up-, perhaps representing *ex-upo-) of PIE root *upo- "from below," hence "turning upward, upward, up, up from under, over, beyond" (cf. Sanskrit upa "near, under, up to, on," Greek hypo "under," Gothic iup, Old Norse, Old English upp "up, upward," Hittite up-zi "rises"). Used as a prefix and in various combinations.
The original meaning is now obscured in many words from Latin ( suggest, suspect, subject, etc.). The prefix is active in Modern English, sometimes meaning "subordinate" (as in subcontinent, first recorded 1863) or "inferior" (a sense first attested 1963).
Below; under; beneath: subcutaneous.
Subordinate; secondary: subinfection.
Less than completely or normally; nearly; almost: subfertility.
A prefix that means "underneath or lower" (as in subsoil), "a subordinate or secondary part of something else" (as in subphylum.), or "less than completely" (as in subtropical.)