I needed to look up a French word in the dictionary earlier (the word was ahurissant) and the English translation given was 'flabbergasting'. I think that flabbergasting is a rare form of the verb to flabbergast - although the infinitive flabbergast isn't used much either. Having said that, I've just put flabbergasting into Google News page and there are several instances of it, mostly in US papers.
The verb is usually seen in the form flabbergasted, and even then it is used as an adjective eg I was flabbergasted. Its origin is unknown, although the OED suggests it may have come from a regional dialect, possibly Suffolk or from Perthshire in Scotland. Another suggestion of the OED is that it is a made-up word from flab or flap and aghast.
The OED also has the rather nice word flabbergastation, the act of flabbergasting or the state of being flabbergasted. It also has a lovely variant of flabbergast, sadly only a nonce-word -- flabberdegasky.
