Green’s Dictionary of Slang

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

1901		‘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.
1912		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.
1924		G. Henderson Keys to Crookdom 419: Squeal. To talk, tell, inform [...] fanning.
1937		J.M. Cain Serenade (1985) 150: We were fanning along.
1958		E. Gilbert Vice Trap 40: I’d come over to fan with Sand-o a little.

2. (UK und.) to concoct a ‘tale’ in pursuit of a confidence trick.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 4: Fanning: Telling the tale (false story).