to put a negative or stupid comment on a persons facebook thread. Particularly when the topic or "thread" is upbeat and funny. A threadkill will usually bring a halt to the conversation. A threadkill can be used as a noun or a verb.
facebook topic starts: What a great day the weather is awesome
Comment 1: gorgeous 80 and sunny
Comment 2: We have a little rain but life is good!
Threadkill: I can't go outside ever because I get sunburned and I have allergies