flab·ber·gast
Pronunciation: ‘fla-bər-ˌgast
Function: transitive verb
Etymology: origin unknown
Date: 1772

: to overwhelm with shock, surprise, or wonder : DUMBFOUND
synonyms see SURPRISE
–flab·ber·gast·ing·ly \-ˌgas-tiŋ-lē\ adverb
…
Concise Oxford English Dictionary
flabbergasted
adjective informal surprise greatly.
→ headword
flabbergast
‘flabəgα:st
■ verb [usu. as adjective flabbergasted] informal surprise greatly.

C18: of unknown origin.