1. Crazy. A common colloquialism in Britain since the mid-1960s (it seems to have existed in restricted use since the 1920s), bonkers has more recently been adopted by American teenagers. The inspiration behind it is uncertain, but it may refer to a bang ('bonk') on the head.

2. fun. Influenced by the associated notions of wild, excessive and unrestrained, this adapted sense of the term has been popular among adolescents since 2000.

We had a bonkers day out.