A Birthday Sandwich is a sexual innuendo for a man laying on his back, a woman straddling him, and another man filling the final hole of said woman. Said sandwich must be received on birthday.
A Birthday Sandwich on any other occasion would be commonly known as a Three-Way.
Something someone may be said to eat if they were to:
1) fall unmercifully in a painful crash
2) lose miserably at a game or contest
3) fail terribly at an assigned task
1) Holy christ, that guy just ate a shit sandwich!
2) What Bill Buckner did for the Red Sox. Bill ate a shit sandwich.
3) The Repubclican guard feasted on shit sandwiches