I don't know anything about that - A way of telling someone you think they're a complete waste

A way of telling someone you think they're a complete waste of space and you're going to make them feel it.

'I don't know anything about that' said when your cousin said he could beat up a superhero.

'I don't know anything about that' after your friend said he could run a marathon in 30 seconds.

'I don't know anything about that' when your dad said he could eat a whole cake in one bite.