First found in the StrongBadEmail episode StrongBadEmail/the bird, this refers to a style of flipping someone off using both of your middle fingers. It has since become the main attack of Stinko Man.

It is also an easter egg in the StrongbadEmail StrongBadEmail/2 Emails (Seen by clicking on 'double the fun' after strongbad says "double the fun").
It's called Double Deuce-bluemint Gum!

Strong Bad's action figure from StrongBadEmail/action figure has Double Deuce Action.

Stinkoman StrongBadEmail/japanese cartoon has double duece action. Just pass over the word email on the anime main page.

In StrongBadEmail/big white face, Strong Bad gives Strong Sad a single deuce (like the double deuce, but with only one finger.

The Grape Nuts Robot that Bubs made in StrongBadEmail/personal favorites is programmed to say "double deuce".

One of Strong Bad's techniques to rid himself of the office dullard (StrongBadEmail/dullard) is the no-look single deuce.