Green’s Dictionary of Slang

1. the mouth.

1860		Glasgow Herald 20 Apr. 4/3: Tom got a hot ’un on the whistler, which shook his ivories and turned on a fresh tap.
1889		Times-Democrat (New Orleans, LA) 9 July 3/6: Prize Ring Slang [...] ‘kisser,’ ‘grubber,’ ‘trap,’ ‘whistler,’ ‘ivory-box,’ the mouth.
1903		(ref. to 1810s–50s) Bulletin (Sydney) 23 July 21/4: The mouth was variously known as ‘potato trap,’ ‘kisser,’ ‘kissing organ,’ ‘ivory box,’ ‘oration trap,’ ‘whistler,’ ‘sucker’ – all are expressive terms.

2. an audible breaking of wind.

1977

J. Sayles Union Dues (1978) 13: You let off one of them whistlers from your business end near a methane pocket an we’ll all be playing in the harp section.

3. (US gay) a fellator.

1972

B. Rodgers Queens’ Vernacular.