- bi- prefix meaning "two, twice, double, doubly, once every two," etc., from L. bi- "twice, double," from Old L. dvi- (cognate with Skt. dvi-, Gk. di-, O.E. twi- "twice, double"), from PIE root *dwo- "two." Nativized from 16c. Occasionally bin- before vowels; this form originated in French, not Latin, and might be partly based on or influenced by L. bini "twofold" (see BINARY (Cf. binary)).
Etymology dictionary. 2014.