This is a mash up of “get your act/shit together” (get organized) and “get off your ass” (stop loafing and get to work). Both idioms direct someone to get moving, invoking a congruent conflation. Also ass and act are three letter words that have a similar sound. Finally, the lower half of the body is at work here in both phrases. Many thanks to Robyn Pietrucha for blurting this one out and passing it along!