To become angry. I didn't tell Dad that I scratched up his car because I knew he'd get mad. You lied to me, so of course I got mad!
get mad (at something)
to muster all one's physical and mental resources in order to do something difficult. Come on, Bill. If you're going to lift your end of the piano, you're going to have to get mad at it. The sergeant keeps yelling, "Work, work! Push, push! Come on, you guys, get mad!"