also bi-monthly, 1846, "happening once in two months, every two months," also "occurring twice a month," a hybrid from bi- "two" + monthly. Bimensal in the same sense is attested in 1670s.
Entries linking to bimonthly
bi-
word-forming element meaning "two, having two, twice, double, doubly, twofold, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, dis-, Old English twi-, German zwei- "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 Latin bini "twofold" (see binary). In chemical terms, it denotes two parts or equivalents of the substance referred to. Cognate with twi- and di- (1).
monthly adv.
"once a month, in every month," 1530s, from month + -ly (2).