A barfday reset is any event when you throw up, vomit, barf, puke, etc. You've expelled your stomach contents, you now have a new barfday.
I was hoping to celebrate my barfday next Thursday, but last night I had the grossest barfday reset ever. Chowder, EVERYWHERE.
My 5yr old son just reset his barfday this morning, I'm going to have to work from home today.