dumbfoundering

/dʌmˈfaʊndərɪŋ/
adjective
  1. Causing great confusion or amazement; bewildering.
    • The magician's dumbfoundering trick left the audience speechless.
    • The complexity of the puzzle was dumbfoundering to even the smartest players.
    • She found the sudden change in the rules absolutely dumbfoundering.