- bi-This prefix denoting ‘two’ or ‘twice’ causes much confusion in words such as biweekly, bimonthly, etc., because it is used to mean (for example) ‘twice a week’ as well as ‘every two weeks’. Rules do not help because they are not universally followed; it is best to avoid the problem by using terms such as twice a week and twice-weekly on the one hand, and every two weeks and fortnightly on the other. Bi (without the hyphen) is a recent informal shortening of bisexual.:
• There are no gay or lesbian or bi or tranny bookstores in Lisbon —Curve Magazine, AmE 2002 [OEC].
Modern English usage. 2014.