nonplusser

From nonplus (“to bewilder or perplex”) +‎ -er.

nonplusser (plural nonplussers)

1. (rare) Something which causes one to become nonplussed or confused.
   • 1940, Sydney Horler, Tiger Standish Steps on It, London: H&S Services Yellow Jackets, published 1944, page 88:

     This might have been a nonplusser to the average individual, but Siandish, already prepared, gave a ready reply.

• “nonplusser, n.”, in OED Online ⁠, Oxford: Oxford University Press, launched 2000.