A nade-tard moment is in reference to Call of Duty. This is when a player looks around a corner for about 10 second, and thinks the coast is clear. Right as he starts to pull the pin on a grenade, an emeny jumps around the corner and kills the grenadier.
Guy: Ya, I died like 20 times this round.
Guy2: Dude! How did you do that?
Guy: Well, I had a few nade-tard moments. They got me every time!
Guy2: Ha ha!. That blows(no pun intended)