From WordNet (r) 3.0 (2006):
nonplus
v 1: be a mystery or bewildering to; "This beats me!"; "Got me--
I don't know the answer!"; "a vexing problem"; "This
question really stuck me" [syn: perplex, vex, stick,
get, puzzle, mystify, baffle, beat, pose,
bewilder, flummox, stupefy, nonplus, gravel,
amaze, dumbfound]