(American)

1. excellent, attractive. A vogue term of appreciation or approval in use among adolescents since the early 1990s, first associated with the slacker and grunge subcultures as well as the language of college students. Boff is a variant form.

'She's buff!' (Sneakers, US film, 1993)

2. physically fit In both senses, the word had been adopted by UK adolescents by 2000. The superlative form is buffest.