(a body part) is killing (one)
A particular body part is causing one pain. Ugh, my head is killing me—do you have any Advil? Phil's back is killing him, so he went back to the hotel to lie down.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
something is killing someone
Fig. something is causing someone pain. Wow, my feet are killing me!
See also: killing
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.