from reddit(coldAntique291): "Flabbergasted" is a made up 1700's word combining "flabby" and "aghast". Meaning utterly shocked. No real "flabber" get "gasted".

I asked AI and it told me this expression is a play on the word "flabbergasted" and it's a funny way to say "feel flabbergasted", since the original word is a bit too long. So neither "flabber" nor "gasted" here has an actual meaning separately.