from Latin, earlier dui-, related to Greek di- 'two' and Sanskrit dvi- 'doubly, having two'.
The meaning of bimonthly (and other similar words such as biweekly and biyearly) is ambiguous. Bimonthly, for instance, can mean either ‘occurring or produced twice a month’ or ‘occurring or produced every two months.’ So, if an organization holds bimonthly meetings, over the course of a year, will it meet six times or twenty-four times? The only way to avoid this ambiguity is to use alternative expressions like every two months and twice a month. See also biennial (usage).