brown trout, n. — Green’s Dictionary of Slang

brown trout n.

[it ‘swims’ in the lavatory bowl]

excrement; thus brown trout pond n., a lavatory bowl.

1992		Bentley & Corbett Prison Sl. 89: Brown Trout [...] excrement.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Brown Trout (n): excrement.
2005		Roger’s Profanisaurus in Viz Apr. 47: brown trout pond n. The lavatory bowl.

fish for brown trout (v.)

(US gay) to have anal intercourse.

1989–2003

R.O. Scott Gay Sl. Dict. 🌐 anal intercourse: [...] Syn: fish for brown trout.

sling trout (v.)

(US prison) to throw excrement or urine over another prisoner.

1992

Bentley & Corbett Prison Sl. 89: .Slingin’ Trout Throwing body waste on the guards passing the cells.