get (one's) back up

1. To become angry, hostile, defensive, or irritable. John got his back up when his parents brought up the subject of college. My dad always gets his back up during election season. You're not in trouble or anything, so please don't get your back up. I'm just looking for information.

2. To cause one to become angry, hostile, defensive, or irritable. Election season always gets my dad's back up. Few things get my back up like hearing about a poacher killing an endangered animal. Why would you bring up tax reform around Uncle Ed? You know that gets his back up.

get back up

To return to an upright or standing position, especially after having fallen or after having sat or lain down. I know you just sat down, but I'm afraid I'm going to have to ask you to get back up—those are not your seats. He's taken a beating tonight, and this might be it for the champ. No, wait, he's getting back up! I had a hard time getting back up after I slipped on the ice.

get someone's back up, to

To make someone angry. The expression alludes to the behavior of the domestic cat, which arches its back when it is attacked by a dog or is otherwise annoyed. This term began in the early eighteenth century as to put or set up the back. By 1864 it was, “He goes his own way . . . if you put his back up” (Sunday Magazine). See also get someone's dander up.