Green’s Dictionary of Slang

strap on v.

[coarse use of SE]

to have sexual intercourse.

[US]F. Hilaire Thanatos 56: Why don’t you strap a little on me?
[US](con. early 1950s) J. Ellroy L.A. Confidential 9: Ordered Mrs. Bowron to ‘Strap on, bitch’.