drop the soap (in the shower), v. — Green’s Dictionary of Slang

drop the soap (in the shower) v.

[orig. gay use, but also as a semi-joc. warning from one self-proclaimedly heterosexual young man to another, I wouldn’t drop the soap while he was around]

to make oneself available for anal penetration.

1979		Maledicta III:2 231: He also may or may not know the following words and expressions: [...] bend over (Navy: drop the soap in the shower).
1993		M.B. ‘Chopper’ Read How to Shoot Friends 119: He used two to four $50 caps of heroin per day and so he was flat out dropping the soap in the shower to pay his way.

In phrases

play drop the soap (v.) (also play pick up the soap)

(US) to indulge in homosexual activity; also attrib.

1953		L. Uris Battle Cry (1964) 135: I hear [...] he’s been playing drop the soap with Bryce.
1972		B. Rodgers Queens’ Vernacular 22: Out to sea, boredom is relieved by playing drop [pick up] the soap.
2005		M.B. ‘Chopper’ Read Chopper 4 92: The drop-the-soap competitions had been getting completely out of control.
2024		J. Meades Empty Wigs (t/s) 514: [W]e afterglowed in my emperor size bed [...] We’d played drop the soap for a good hour!