A somewhat sarcastic nickname given to somebody who just gave a random scientific fact that might have something to do with the current conversation.
Originated from the original Doctor Science, a character belonging to comedian Dan Coffey, who "knows more than you do", has "a masters degree! In Science!", and would answer questions about random yet silly science with bogus yet silly answers (and even had a television show for a season or two in the states).
Q: Dear Doctor Science, if the speed of light is 186,000 miles per hour, what's the speed of dark?
A: Well, since dark is broken light, it doesn't really have a speed; it just kind of sits around until somebody turns on the light.
Somebody who either isn't, or somebody who can only be trusted to help you with that one menial task that would save you about 20 minutes of walking around. Invariably spoken with a slight hesitation before the term.
Johnson was a, uh, big help today as he told me when the light stopped blinking on the router.