Haha, What?

"Haha, What?" is a term used to opt out of adding anything meaningful or providing commentary to a conversation and instead purely state one's confusion towards the current topic in either a rudely sarcastic or playfully jesting manner

For example:"'I am thinking about trying out for the football team, what do you think?'""'Haha, what?'"or"'What if everyone just turned into a pear and we could make fruit cocktails out of our pee pee whenever we wanted?'""'Haha, what?""Haha, Yes." is a term used in a similar manner, except to state one's agreement, potentially in an inronic manner, but nevertheless still just as disinterested and sarcastic in tone.

For practical usage, tone of voice and delivery are important for either of these situations.

With enough practice you might even be able to pull off saying just "Haha" as a response, but having all of your implications come across to everyone within range.