It was Yudkowsky's colloquial term for hard takeoff:

https://www.lesswrong.com/posts/tjH8XPxAnr6JRbh7k/hard-takeo...

Usually refers to a sudden increase in AI intelligence to super intelligence, i.e. the singularity. Basically an exponential increase in capability.

It may also be a reference to the (comparatively ancient) "Pentium go F00F" bug.

https://en.wikipedia.org/wiki/Pentium_F00F_bug

Usually? How many times has that happened?

Explosive ignition of a fire.