Where has he even heard the term before?

I was in the backyard this afternoon tidying things away when I heard Tom behind me.

Hey Mum, guess what?  I'm a coke head!!

I swung round in horror, taken aback and mighty confused, only to be presented with this...

... I burst out laughing, and he was very pleased with my reaction, little realising that the two of us were laughing at very different things!