from The American Heritage® Dictionary of the English Language, 4th Edition
- transitive v. To cause to be overcome with astonishment; astound. See Synonyms at surprise.
from the GNU version of the Collaborative International Dictionary of English
- transitive v. To astonish; to strike with wonder, esp. by extraordinary statements.
from The Century Dictionary and Cyclopedia
- To overcome with confusion or bewilderment; astonish, with ludicrous effect; confound: as, the news completely flabbergasted him.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- v. overcome with amazement
Origin unknown.(American Heritage® Dictionary of the English Language, Fourth Edition)
Origin uncertain. Hotten says it is from Old English; Whitney and Smith suggests flabby or flap (strike) + gast (astonish); The Imperial Dictionary connects it with flabber (related to flap, to strike) + the root of aghast, and notes that flabagast may have been the root (to strike aghast); first documented as slang in 1772;  Cassell gives it as dialectical from Suffolk, from flap or flabby + aghast, possibly related to Scottish flabrigast (to boast) or flabrigastit (worn out with exertion); Smith relates it to flab (to quake) or flap (to make a flap over something) + Middle English agasten (to terrify), and relates it to aghast, ghastly and ghost (Wiktionary)