buy (something) on tick

To purchase something on credit. "Tick" is short for "ticket," a note that showed one's debt (although this practice is less common today). Primarily heard in UK. I've already spent so much on this trip that I had to buy some last-minute purchases on tick. Well, if you need groceries and won't get paid for another week, just buy them on tick.

get, buy, etc. something on ˈtick

(old-fashioned, British English, informal) get food or other goods and pay for them later: You can only buy things on tick in small shops where they know you well. Tick is probably a short form of ticket and referred to the written note (an IOU) given by a person borrowing something to the lender as proof that they would pay them back or return the goods.