flabbergast
To overwhelm with bewilderment; to amaze, confound, or stun, especially in a ludicrous manner.
transitive
He was flabbergasted to find that his work had been done for him before he began.
Where it comes from
The origin of the verb is uncertain; possibly dialectal (Suffolk), from flabby or flap (“to strike”) + aghast. The word may be related to Scottish flabrigast (“to boast”) or flabrigastit (“worn out with exertion”). The noun is derived from the verb.
Adapted from the Wiktionary entry flabbergast (CC BY-SA 4.0).