aitchbone (n.)

"rump-bone in cattle," also the cut of beef which includes this, late 15c., a misdivision of Middle English nache-bone (see N), from nache "buttocks" (c. 1300), from Old French nache, nage "the buttocks," from Medieval Latin *natica, from Latin natis "buttock," from PIE *not- "buttock, back."