Green’s Dictionary of Slang

fan v.4

(US) to calm someone down.

[US](con. 1930) D. Wells Night People 50: If we had been on good terms with the rooming house and restaurant the last time, he wouldn’t be gone long, but if he had to rub them down (or ‘fan’ them, as Buddy Tate would say), he would be gone just about all day. [Ibid.] 117: Fan. mollify.