Same word as quickwit:

acute, agile, alert, apt, astute, brainy, bright, brilliant, canny, clever, facetious, humorous, intelligent, keen, knowing, nimble-witted, on the ball, penetrating, penetrative, perceptive, prompt, quick, quick on the draw, quick on the uptake, ready, savvy, sharp, sharp as a tack, sharp-sighted, sharp-witted, shrewd, slick, whiz, wired, wise, witty