dirty pool

(idiomatic, transitive, UK, Australia, informal) To treat (someone) in an unfair manner; to cheat or betray.

(slang, derogatory) Sleazy or underhand behavior.

Morally unclean; obscene or indecent, especially sexually.

A phrase that covers a lot of ground depending on tone and context. In its everyday sense, dirty business refers to underhanded, unethical, or morally compromised dealings — the kind of shady conduct that makes people uncomfortable. In more explicit slang use, it's a euphemism for anal sex. The beauty of the phrase is how cleanly it inhabits both registers, letting speakers be deliberately ambiguous. Which meaning is intended usually becomes clear quickly from context and the company you're keeping.