When someone says they are going to not do something ever again, and break that statement within the same day.
The term comes from the zombie survival game DayZ, in which people tend to get very angry upon death and state that they will never play the game again. They almost always end up playing the game again the same day.
"Oh dammit I died again in DayZ, I'm never playing this stupid game again!!"
"It's just The DayZ Effect! You'll be playing it again within an hour."
When you run with a group of people on DayZ for a fat minute, then switch up to play with someone new for a few days while appearing offline, only to have your friends join the party on alt accounts to find you and said person together.
Squad: I can't believe you, I thought you were rollin with us?
Victim: I am, I'm just---
Squad: I don't wanna hear it. I can't believe you're DayZ Cheating on us.