Green’s Dictionary of Slang

1. (man-to-man) fellation; modern use, intercourse under the armpit, generally a homosexual practice.

1785		Grose Classical Dict. of the Vulgar Tongue n.p.: Bagpipe, to bagpipe, a lascivious practice too indecent for explanation.
1890–1904		Farmer & Henley Sl. and Its Analogues III 107/2: gamaruche..Verb (venery).—To irrumate; to bag-pipe (q.v.). [Ibid.] 373/2: huffle, verb.(venery).—To bag-pipe (q.v.).
1972		B. Rodgers Queens’ Vernacular.
1989–2003		R.O. Scott Gay Sl. Dict. 🌐 bagpipe: Sexual intercourse performed on the armpit of a partner until an ejaculation is reached.
1999		Hip-Hop Connection Dec. 45: ‘Buddy’ which involved Jimmy, Jenny, troilism, bagpiping and ball torture.
2000		Desdemona at www.asstr.org 🌐 Cherry knew before she was ten that a nooner was sex in the afternoon. And by twelve she could explain that bagpiping was a man fucking your armpit, and knee-tremblin’ was when you had sex standing up.

2. intercourse with the penis rubbing between the breasts.

1899

Horn Bk 135: Bag-Piping. The man stands up, his backside supported by the edge of the bed, and the woman is on her knees before him... She places the beloved prick between her two breasts, the point coming under her chin, and she presses her two full globes with each hand, looking with seet adoration at her lover, who is moving his arse as if he was in her cunt or her bottom [Simes:DLSS].