A moment when you get so freaking annoyed with someone that you send them a multiple page letter/email/text message describing how much you've had to put up with them in the past, how they annoy you, and how you never want to talk to them again. Subtle insults are key. The difference between a Luke Moment and just totally ripping on someone is that you remain slightly nice with only subtle insults. Then the person replies and that's when you let it all loose.( :0 )
Sometimes, the friendship may restart a few days after the Luke Moment if the person "recieving" the Luke Moment is absolutely retarded.
The phrase "Luke Moment" was coined when a dude named Luke Kirstein sent 3 of these letters to the most annoying people on the planet Earth. Their names are withheld for security purposes :].
Harmony: *crying* The boy I liked just totally yelled at me on myspace! I'm so sad!
Megan: It's ok, he probably just had a "Luke Moment".