The English language is fun in itself sometimes. I recently got an email with a word I’ve not seen or heard: biyearly. Ok is that every two years or twice a year? well, at it turns out the answer is yes. Same goes for bimonthly or biweekly. It means both twice a week or every two weeks.
If you look at wordreference, they hint at better words to use:
fortnightly – meaning occurring every two weeks,
semiweekly – meaning twice a week.
Ok, so I had a 100% chance of getting it right, but now I have a crusade to squash the use of bi for time. ;p