A term commonly used to indicate that someone (often the speaker) has come off the loser. This phrase can be applied to competitions such as video games and sporting events, as well as such things as tests or writing papers.
How was the econ test?
Dude, it totally ate my lunch...
Did you see Manning just get sacked?
Yeah, that linebacker totally ate his lunch!
I need help in bottom lane, this Warwick is eating my lunch!