Managed to teach my 2 yr old the phrase and now I regret ever opening my mouth to tell him I needed to clean his penis.
He's had an infected one recently, for context, so we've been telling him he needs to keep his hands out his trousers/nappy to help with it. He then didn't want me to change his nappy because he was having a 2 yr old strop, and the phrase just slipped out my mouth.
DH didn't help the matter by laughing, which probably caused DS to repeat it.
Ah well. At least he won't be around anyone in public to repeat it now.