Half cider and half beer. (The beer is usually lager but some people put bitter in it).
Those who drink it (who are almost exclusively young males) claim that you can get drunk quicker than by simply drinking cider or beer on their own.
Many pub landlords refuse to serve it because the truth about the drink is that people don't get drunk a lot quicker, they just throw up much sooner (usually all over the floor of the bar).
Couldn't touch one these days with a bargepole, but as a youth in the late 70s/early 80s we had pink medicines... kind of like nat_84's Snakebite & black. Shot of vodka (or 2), measure of blackcurrent, small bottle of black label pils then topped up with cider to the pint mark. Oh dear oh dear oh dear LOL
A proper snakebite was made up of
Carlsberg Special Brew and Merrydown cider, both of which are pretty potent on their own, but mixed , sent you totally daft.