Green’s Dictionary of Slang

1. a beating, a thrashing.

1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1904		Number 1500 Life In Sing Sing 248: Fanning. spanking.
1934		H. Miller Tropic of Cancer (1963) 265: A cop came along and [...] gave us such a sound fanning that we were up and on our feet in a jiffy.
1947		I. Shulman Amboy Dukes 108: Shut up [...] or you’ll get a fanning.

2. (US Und.) a pickpocket’s preliminary running of their hands over a victim to find a wallet or bankroll.

1904		Number 1500 Life In Sing Sing 248: Fanning. Locating purse.
1924		G. Henderson Keys to Crookdom 404: Fanning. Locating a purse.
1949		Monteleone Criminal Sl. (rev. edn).

3. a search of a person, usu. for weapons.

1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 438: fanning, n. A search, or frisk, by the police or prison guards.
1938		D. Runyon ‘The Brakeman’s Daughter’ in Runyon on Broadway (1954) 509: Big False Face gives The Humming Bird a quick fanning.