A joke. When overnighting at a friend/girlfriend's place and you bathe or shower the next morning,you inform your host that you took the liberty of using their back brush.In most cases,the host will advise you that they don't own a back brush. You then ask,"Well,what is that loop-shaped brush in the container on the bathroom floor?" After they quit laughing,they'll say,"That's the fuckin'toilet brush,dumbass!" Guaranteed giggles every time.
When you "borrow" something edible/consumable without someone's permission, that probably, won't be missed, with no intention of ever, ever, giving it back.
If you survive some situation where you would have otherwise did were it not for some miracle of God then your entire life after that point is said to be borrowed time.
A man walks across a street without looking both ways when he accidently steps on a bottle, that happened to be rolling by, and falls straight down. Exactly half a speeding truck passes in front of him, only missing by inches. This man is living on borrowed time because if it hadn't been for the bottle rolling by at that exact moment he would have been smashed like a caterpillar eating your grandmas favorite rose bush.
Taken from the movie "Gattaca". The term is associated with a detested segment of society; one of those who refuses to play the hand that he was dealt. Most commonly known as a borrowed ladder, or a de-gene-erate.