A dirty shot is similar to a normal shot of alchohol but involves mixing things together which should not normally be consumed at the same time. Usually, the drinker has to be quite intoxicated already to agree to drinking such shots.
"I was being plied with dirty shots last night, I was completely wasted and even had a vodka and actimel one!"
The Dirty Shot is one of the easiest ways to ruin somebody's night at a party. When you notice your rival has had a bit too much to drink, you replace his/her shot glass with one full of cum. Enjoy as you watch first looks of confusion and then looks of horror.
Bro #1 'dude did you see that asshole matt last night'
Bro #2 'ya i can't beleive that douche took a dirty shot'