Describing a state where a person's brain can't process information quickly. This is not to say that they are outright stupid, just slower than average. Origin comes from field of computing where threads represent units of processing in a system capable of concurrent processing.
How single-threaded was Brian in that meeting today? He was not keeping up at all.
Totally. Anthony was much better - across everything that was going on. He's one multi-threaded mo-fo.