Ever heard the expression, "you'll thank me later"?

I have used that term many times in my life, but it has nothing to do with food. Usually it only referred to sex. Such as doing a favor for friends and they want to thank you now for your service, but I always tell them, "that's okay, I'll let you thank me later",,,,by the way,,, I need to thank you later for last night.

Its just a saying, as you say, an "expression". You aren't meant to take the phrase literally but metaphorically.

All it means is that at some stage in the future you will be pleased that you weren't able to do something you wanted to do at the time but were prevented by another person.

I've heard it before but I don't remember ever thanking someone later.

At this point I don't feel like thinking anyone for anything I've had a rather bad day

Yes, in the TV show "Monk" - Mr. Monk uses it as a catch phrase.

Yep

Its just an expression,when you tell someone break a leg you dont want them actually breaking thier leg.