a few sandwiches short of a picnic
A person who thinks a picnic is just a fancy lunch and forgets to bring enough food, then acts like it’s not their fault.
My cousin forgot to bring a sandwich and just ate a bag of chips like it was a picnic tradition.
My mom brought one sandwich and said it was enough for six people. It wasn’t.
My brother showed up with a sandwich and then forgot to bring the rest. He called it a half-assed picnic.