McCarthy's works are filled with obscure words, but this one is particularly strange, and prior to me posting this question, almost constitutes a Googlewhack.
I'm reading the Kindle edition and so it could even be a misprint.
The passage is this:
The black wiped his eyes with one huge hand. Stories of the days and nights writ there, the scars, the teeth, the ear betrun-cheoned in some old fray that clung in a toadlike node to the side of his shaven head.
The word is betruncheoned. The hyphen is misplaced (see comment by @GEdgar).
To truncheon something is to hit it with a truncheon (a type of club used by the British police). Thus a betruncheoned ear is one that has been hit by a truncheon.
NOTE
To understand why the word is "betruncheoned" and not simply "truncheoned", the following is relevant How does the "be-" prefix change the words to which it is applied? How did it come about?