Head-Spinningly Meaning

(idiomatic) incredibly

Example: 2012 May 20, Nathan Rabin, “TV: Review: THE SIMPSONS (CLASSIC): “Marge Gets A Job” (season 4, episode 7; originally aired 11/05/1992)”[1], The Onion AV Club:
  â€œMarge Gets A Job” is just about perfect. There is not a wasted sequence or unfunny gag. It’s head-spinningly smart in a way that doesn’t call attention to itself—like setting the retirement party at The Spruce Caboose, a giant, unwieldy, trainwreck-themed eatery whose name and conceit are brilliant parodies of Howard Hughes’ giant wooden airplane, the Spruce Goose.