Noun. A very smart person. Lit. a “lightning comprehender,” someone who gets things in a flash. A compound word consisting of Blitz (lightning) and a derivative of gneißen, an archaic Austro-Bavarian verb meaning “to comprehend.” Which would be a great compliment, if you actually meant it.
In Austria, however, it is more often applied sarcastically, to denigrate someone who is quite the opposite. But English-speakers beware: Do not equate this dig with expressions like “smartass” or “wiseacre,” which imply impertinence and are better translated as oberg’scheit (lit. “overclever”). Calling someone a Blitzgneißer simply means that person is a bit slow on the uptake — except, of course, in the rare cases it’s used sincerely.
Note that it’s entirely possible to be an oberg’scheiter Blitzgneißer, and the two, in fact, often go hand in hand.