from Latin, 'doubly, having two'; related to Greek di- 'two'.
The meaning of bimonthly (and other similar words such as biweekly and biyearly) is ambiguous. Such words can either mean ‘occurring or produced twice a month’ or ‘occurring or produced every two months’. The only way to avoid this ambiguity is to use alternative expressions like every two months and twice a month. Biannual and biennial are also confusing: biannual means ‘occurring twice a year’ and biennial means ‘occurring every two years’.