Green’s Dictionary of Slang

n.f. n.

[abbr.]

1. no fool.

[UK]J. Ware Passing Eng. of the Victorian Era.

2. (US campus) no fun.

[US]Eble Campus Sl. Apr. 3: n.f. – [...] ‘no fun’ usually used in a teasing or derogatory manner: You’re not staying in to study are you? Man, n.f.