British Slang Term of the Day: "On a Nap"

I love British wit, and this one takes the crumpet. I saw a video where a young British woman was confronting a guy who was cheating on his wife. She said "So, why are you on a nap anyway?" Despite the bad bloke, I had to laugh.

 It hit me right away, but I rewound the vid just to make sure I heard it right. "On a nap." That's classic. Obviously it means "fooling around," or as we sometimes say here, "Getting some 'strange,'" but it's so elegant and almost charming, and 100% British. Those mad Brits...they're amusing. Got me good, mate.