"Arse Pirate" is a witty combination of the words "arse" (rear end) and "pirate" (if you don't know, you're an idiot). The phrase is highly versatile and consequently you will find it appropriate the next time you witness a rimming, or an act of homosexual lovin'.
1. Where it simply means gay man - "That arse pirate sure is going for it"
2. By a child (v. rare) "Mummy, when I grow up, I want to be an arse pirate"