Slang idioms

"I am sorry that I punched you in the face when you walked through the door last night, Bill. It was just a knee-jerk ...... I wasn't expecting you you said you were going to be out of town. I heard the knob rattle at 12 o'clock at night. I went to check it out, the door opened, and I swung. I thought you were a burglar," Matt said.