Learn English – Origin of “scumbag”

It often implies that you have made a serious mistake, but more strictly it indicates that something you have done (or some turn of events) has left you looking embarrassed or foolish:

Murray knows all about the Chilean 12th seed — he practised with him last week — and the British number one realises he could be left with egg on his face if he leaves Gonzalez an easy, mid-court ball.

Where it came from (source):

I know of two suggestions for where it came from. The late John Ciardi suggested an origin in the lower-class and more rowdy kind of theatrical performance, in which an incompetent actor would have been pelted with eggs and forced off the stage. The other is that it was a comment on a minor social gaffe at a meal, when poor manners or sloppy eating left egg around your mouth.

And also (source):

From the embarrassment suffered if the yellow yolk is on ones lips or beard after eating a soft boiled egg in one of those egg cups, a favorite breakfast of the upper crust... Yellow egg shows up especially well on a beard or mustache.

It's an English adaptation of a Latin saying:

De gustibus non est disputandum.

Meaning literally regarding taste, there is no dispute. The phrase seems to be of medieval origin. The origin is accepted as Scholastic writings because of the grammar, which is atypical. A more faithful Latin rendering of the phrase might be:

De gustatibus non disputandum.

There's some uncertainty about whether gustus (gustibus) or gustatus (gustatibus) is more appropriate.