Drink Surfing
The act of ingesting half empty drinks or abandoned drinks in a bar when you are drunk and either too poor or tightfisted to buy a drink.

Usually has a number of bad consequences such as drinking spiked drinks, Getting confronted by the original owner of a drink. getting herps? and most likely ending up mangled in a few short minutes, depending how skilled a surfer you are.
Drunk Person 1: man drinks are well expensive here, £5 for a bottle of beer you gotta be kidding me.

Drunk Person 2: hey check it out "Drunk Person 1" theres a few beers over there that have been abandoned.

Drunk Person 1: Plan we should start drink surfing.

Drunk person 2: Sweet why didn't we do this before tonight

Drunk Person 1: this beers a little warm but man tastes nice been free.

Drunk Person 2: I found some more and this glass of something think its rum and coke. Pfftt tastes weak as hell.

Drunk person 1: haha can't complain, I hope no-one notices.

Later that night....

Drunk person 2: #phonecall to "Drunk Person 1" "Yo where did you end up, I just got out of hospital." - drink surf fail.