It's basically like "pwnd" or "owned". Yell it when someone trips down a flight of stairs, or something.
Alledgedly from Halo.
Primus: *falls down stairs*
Secunda: Seized, asshole!
Primus: *is pushed down stairs by Secunda*
Secunda: Managed to seize ya!
to get absolutely shitfaced drunk and stoned
"Me and steve were just fuckin seized that night"