Green’s Dictionary of Slang

fan v.3

[SE fan, a supporter; ult. fanatic]

(US) to converse, to chat, usu. about sport.

[US]‘Hugh McHugh’ Down the Line 36: In a minute the three of them were fanning each other with fairy tales about the goods they sold.
[US]Van Loan ‘McCluskey’s Prodigal’ in Ten-Thousand-Dollar Arm 262: That he did not join in the usual ‘fanning bee’ [ ...] Ball players, like all other performers, talk shop at every opportunity.
[US]G. Henderson Keys to Crookdom 419: Squeal. To talk, tell, inform [...] fanning.
[US]J.M. Cain Serenade (1985) 150: We were fanning along.
[US]E. Gilbert Vice Trap 40: I’d come over to fan with Sand-o a little.