Joe,

A jar is a colloquialism used across much of the British Isles.

Whilst it usually refers to beer, it isn't used in terms of ordering a certain drink, but refers to having a drink in general.

Generally usage is: "Do you fancy a few jars?" (said to a friend)

When you get to the pub, you order the drinks by name (as in, presumably, the rest of the world)

Hope this makes sense.

Please don't 'try' to be cool, just be yourself