bin- combining form Etymology: Middle English, from Late Latin, from Latin bini two by two; akin to Old English twīn twine — more at twine 1.: two : two by two : two at a time < binary > < binate > < binaural > 2.chemistry: bi- 4 < binoxalate > < binoxide > — in some words of which the last constituent begins with a vowel; compare bi-