Quote Originally Posted by Bijo
My limited internet knowledge tells me it regards being banned. But I speculate it might originate in games where people utilize the term 'instagib' to refer to killing someone in a game with an "instant kill" effecting bloody gore.

If you see "instagibbed" on this forum I assume it's about someone who has been "banned instantly".
Correct on both counts. The instabig option allows Moderators to delete both an user account and its post quickly. This is done when an account is clearly inappropriate, for example a spambot-account.