The Dog’s What?

Conversation between 8 yo and Grandpa over breakfast

8yo: I just realised that Weetabix is a word you make by putting Wheat and biscuit together.

Me: Yes. Is that a sort of Portmanteau word?

[Discussion follows about portmanteau words and compound words]

8yo: I’ve got a strange sort of saying.

Me: ummm? What is it?

8yo [carefully, didactically]: The dog’s bollocks. For when something is really good.