Made famous on Seinfeld some years back.
Hit the stink, then the pink, skip the sink. --or--
From the back to the front, on the hunt.
It is considered an offense in many households, and many parties.
Origin: a Sienfeld episode in which George Castanza double dipped a chip, to the dismay of a party-goer.
Party-goer: Did... did you just double dip that chip?
Party-goer: YOU DOUBLE DIPPED THE CHIP!
George: Double dipped? Wha-what are you talkin about?
Party-goer: You dipped the chip, you took a bite... and you dipped again!
Party-goer: THAT'S LIKE PUTTING YOUR WHOLE MOUTH RIGHT IN THE DIP! Look, from now on when you take a chip, just take one dip and END IT!
2. When you have sex twice over a short period of time. Normally this refers to having sex with someone the morning after you had a drunken hook up the night before, but this can also refer to having sex with two different people in the same night.
2. A: Dude I double dipped last night! Amanda and Terri something like 3 hours apart!
B: OH SHIT dude you had an epic night!
A: BEST. NIGHT. EVER.