Like its name suggests, the term "wrong-un," (taken from cricket jargon) means "wrong one."
It can be used as a transitive verb, which is means "dry anal sex," since it leads to major complications if you try to say "dry wrong-unal sex."
As a noun, the term "wrong-un" is one's arsehole, and the term "dry" doesn't come into play.
As a transitive verb: "Hey, Scotty! I heard ya gave ya woman dry wrong-un!"
As a noun: "Hey, I heard you got some on the week-end, Scotty. Do you do her up the wrong-un? Ha, ha, ha, ha, ha!"