To buy something (usually drugs) with an agreement to pay later, like a bar tab.

apparently meaning to keep something's price on a tab..

used by those strange council estate people from prestwich England..

To acquire something immediately but pay for it later. Most often used when buying drugs but quite valid for any kind of purchase.

There are thee possible origins of the term:

* The vendor keeps a list of who owes what, and "ticks" them off the list on payment
* The vendor has to wait, the clock is ticking
* The customer is a filthy parasite, like a tick on a dogs back.