I got a little ache in the differential.You're walking like there's something wrong with your differential.
differential
slang The buttocks. This term comes from the "differential gear" found in motor vehicles. Wow, he's got a great differential—he must work out!
differential
n. the buttocks; the rear (end). (From the name of the device that joins the axle to the driveshaft in motorized vehicles.) You’re walking like there’s something wrong with your differential.