When you pay for goods and services in hard currency such as cheeses or honeybuns; as opposed to paying on the bubble which will eventually get your ass smashed off the yard.
1. Could you put a little wood on the fire, it is getting chilly in here.
2. Chaz has to get to the studenthealth center, he got a little wood on the fire.
Ian "I've never gotten a speeding ticket. Knock on Wood...wait there's no wood around!"
Jed "Oh yes there is, check out The Knock on Wood Card!"
Ian "That's dope, thanks Jed!"