Military slang. The term is used when someone is relating to troops or people standing close together. Also means to haul ass, go go go etc. The results of a violent, messy collision or mishap involving the entanglement of two or more persons.
Alright sweethearts, you heard the man! And you know the drill assholes and elbows!
'assholes and elbows' refers to working hard at a task. It originated in farm work. If a group of field hands is busy and bent over picking crops, then a supervisor looking out over the group would see nothing but "assholes and elbows".
A derivative term with the same origin and meaning is head down, butt up.
Row boss to field hands: Alright that's enough chit-chat and coffee break! It's time to get back to work and I don't want to see nothing but assholes and elbows until this field is picked clean.