1. An extremely powerful weather storm generally classified in one of three classifications (class 1, class 2, or class 3) with class 3 being the most powerful
2. A blizzard of epic proportions, usually resulting in school cancellations
Guy: That killstorm we had the other night really wrecked my yard. I found my grill in my neighbor's driveway, my newspaper in a storm drain, and my cat in the chimney.
Young kid: Dad, I heard there is supposed to be a class 3 killstorm tonight. Alright! No school tomorrow! Looks like you'll have to stay home and babysit me.
Dad: Son of a bitch! I'll call in to work and tell them nature has screwed me over again. Dammit!
Example 3 (From The Simpsons):
Kent Brockman: The National Weather Service has upgraded Springfield's blizzard from "Winter Wonderland" to a "Class 3 Kill-Storm"!
Marge: I don't like the sound of that "class 3".