Bagpipe

[Sonia]

A musical wind instrument, now used chiefly in the
Highlands of Scotland.

[Monty]

To have sex by using the armpit of another person for stimulation of the penis until orgasm. The term makes reference to the Irish and Scottish musical instruments of the same name: bagpipes have an airbag that is inflated by the performer, held under the arm, and squeezed. This term is heard most often among gays, but the act can be performed by hetero- or homosexuals.

[Sonia]

To use the armpit for sexual gratification.

[Monty]

related to or played on bagpipes (= a Scottish or Irish instrument played by blowing through a pipe into a bag) :