GAMMD

GAMMD

An initialism for "go ahead, make my day," an expression encouraging the listener to challenge or defy the speaker in some way so that the speaker can then exact revenge. It is associated with actor Clint Eastwood's Dirty Harry character, who said it in the 1983 move Sudden Impact. If she texted you GAMMD, she's basically daring you to start that rumor about her.
Farlex Dictionary of Idioms.

go ahead, make my day

 
1. Just try to do me harm or disobey me. I will enjoy punishing you. (From a phrase said in a movie where the person saying the phrase is holding a gun on a villain and would really like the villain to do something that would justify firing the gun. Compare this with Keep it up!) The crook reached into his jacket for his wallet. The cop, thinking the crook was about to draw a gun, said, "Go ahead, make my day!" As Bill pulled back his clenched fist to strike Tom, who is much bigger and stronger than Bill, Tom said, "Make my day!"
2. Go ahead, ruin my day!; Go ahead, give me the bad news. (A sarcastic version of {1}.) Tom (standing in the doorway): Hello, I'm with the Internal Revenue Service. Could I come in? Mary: Go ahead, make my day! Sally: I've got some bad news for you. John: Go ahead, make my day!
See also: go, make
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

Go ahead, make my day

and GAMMD
sent. & comp. abb. I’ll be happy if you do something that will cause me to harm you. So, you’re gonna do it anyway! Go ahead, make my day!
See also: go, make

GAMMD

verb
See Go ahead, make my day
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • Go ahead, make my day
  • Go ahead, make my day.
  • AITYD
  • Have I made myself clear?
  • for all problems
  • BFD
  • Did I stutter?
  • between you, me, and the lamppost
  • I don't suppose
  • suppose