Strange one

[BockingBob]

Here's a strange one for you .FLABBER- GASTED. I know I've been it ,I've seen people with it . It makes some people open their mouths in awe.But where the hell does it come from?.

[Quizmonster]

The source is obscure, but it may have originated in one of the English dialects, possibly in Suffolk. It appears to be a combination of flabby or flappy - suggesting weakness/limpness - and aghast - suggesting astonishment. That is, one is made weak at the knees with amazement, as it were.

[Clanad]

To engage in a blatant example of pleonasm, as well as peripherally, iilluminate, only slightly, Q'S indubitable response; another source that quotes OED, states "...Flabbergast . . . First mentioned in 1772 as a new piece of fashionable slang; possibly of dialectal origin; Moor 1823 records it as a Suffolk word, and Jamieson, "Suppl.," 1825, has "flabrigast" to gasconade, "flabrigastit" worn out with exertion, as used in Perthshire. The formation is unknown; it is plausibly conjectured that the word is an arbitrary invention suggested by "flabby" or "flap" and "aghast. An additional contributor sees a connection betwee your flabbergast and gobsmacked but I think that stretching it a bit...

[mallam]

Not really. After being gobsmacked your gob is a-flabber and you are aghast.