a what
This old British phrase is like saying 'don't you think?' but with more class and less personality. It's the kind of thing your grandpa would say before he got drunk.
'You just said the best thing ever,' he says. 'A what?'
'That was the funniest thing I've ever heard,' he says. 'A what?'
'You're my favorite person,' he says. 'A what?'