[analogous to ‘pissed off’] Said of system administrators who have become annoyed, upset, or touchy owing to suspicions that their sites have been or are going to be victimized by crackers, or used for inappropriate, technically illegal, or even overtly criminal activities. For example, having unreadable files in your home directory called ‘worm’, ‘lockpick’, or ‘goroot’ would probably be an effective (as well as impressively obvious and stupid) way to get your sysadmin hacked off at you.

It has been pointed out that there is precedent for this usage in U.S. Navy slang, in which officers under discipline are sometimes said to be “in hack” and one may speak of “hacking off the C.O.”.