WordWeb Online

1. As if struck dumb with astonishment and surprise
   "a circle of policemen stood dumbfounded by her denial of having seen the accident"
   - dumfounded, flabbergasted, stupefied, thunderstruck, dumbstruck, dumbstricken

1. Be a mystery or bewildering to
   "a dumbfounding problem"
   - perplex, vex, stick, get, puzzle, mystify, baffle, beat, pose, bewilder, flummox, stupefy, nonplus, gravel, amaze

See also: surprised

Type of: bedevil, befuddle, confound, confuse, discombobulate, fox, fuddle, throw