Green’s Dictionary of Slang

brown trout n.

[it ‘swims’ in the lavatory bowl]

[1990s+] excrement; thus brown trout pond n., a lavatory bowl.

In phrases

fish for brown trout (v.)

[1980s+] (US gay) to have anal intercourse.

sling trout (v.)

[1990s+] (US prison) to throw excrement or urine over another prisoner.