the Word Detective discusses this in one of his long columns, so I'll just extract the salient bit here:

Dear Word Detective: Here in Canada we are quite used to hearing a lot of British
expressions. However, I was recently in London and heard about six words I am not
familiar with. The first one I will send and ask you to explain is "Gobsmacked." Methinks
it's something naughty. -- Arnie Wachman, via the internet.

Oh, good. You know, I was worried about how I was going to spend the long winter
nights over the next few months. But now I know I can plan on many happy hours trying
to unravel the mysteries of British slang, a species so obscure that even the natives of that
charming isle often haven't the vaguest notion of what they're really saying or where it
came from.

In this case, however, we're both getting off lightly, because compared to such
Britishisms as "toad in the hole" (a mashed-potato and sausage concoction, I've been
told), "gobsmacked" is fairly easy to explain. "Gob" is a very old (about 400 years old,
actually) English dialect word meaning "mouth," probably taken from Gaelic or Scots,
and related to "gab," also meaning "mouth" or, more commonly, "speech." To be
"gobsmacked" is to be astonished or flabbergasted, as stunned as if you had been
suddenly "smacked" (struck) in the mouth. Curiously, "gobsmacked" has only been found
in print as far back as the 1980s, but it's reasonable to assume that the term has been
around for much longer.

"Gobsmacked" isn't really "naughty" in the sexual sense, but it is generally considered a
bit rude and would not be a good choice of words were you to find yourself in certain
social situations (i.e., "Blimey, Your Majesty, I'm gobsmacked" would probably be a
mistake). Other words based on "gob" (such as the verb "gob," meaning "to spit") are
also not considered fit for polite company, and even the relatively benign "gobstopper" (a
type of large hard candy popular with children) would probably raise Her Majesty's
eyebrows.

from http://www.word-detective.com/012000.html

I think that the Word Detective erred a tad, using i.e. when he actually wanted e.g.

aside to wordwobbled: gab can be a variant of gob, so gabsmacked....