A portmanteau of "Karma" and "Armageddon." References "shit hitting the fan" in an extreme way while acknowledging one's contributions to that event.
Specifically, when all the (usually unpleasant) stuff you've done comes back to you at once.
The only way out is to take your licking, batten the hatches, pull back, and deal with the stuff one by one.
Man, my license was revoked for not paying those speeding tickets, my car got the boot from those unpaid parking tickets, bill collectors are calling, work's a mess from my calling in "sick," and my girlfriend found out about the affair when the other girl stopped by our place... Argh, it's karmageddon!
When there is so much karma due, that there is no need for a battle between good and evil. Karma just comes in and sweeps the evil ones off the planet.
The moment when an accumulation of bad deeds catches up to you and manifests in the form of a catestrophic end-of-world type sceanrio. Usually preceeded by a feeling of impending doom.