If you really dont care at all about a person doing/taking something you say "knock yourself out" as in to do/eat until you pass out (if you really want to).
Same as "go for it" or "go nuts".
Person 2: "Yeah, Knock yourself out"
"There is no way I can win this race. I might as well knock myself out!"
"I was about to kick that guy's ass for the way he was talking to my girlfriend, but he knocked himself out already."