What does it mean?
Få blod på tann, which literally means ‘getting blood on your tooth’, is illustrated by Wictionary with a picture of a bear digging into some kind of bloody prey.
Although the expression itself has little to do with actual teeth and blood, Wictionary's image isn’t that far out.
Få blod på tann refers to the special kind of determination that a wild animal gets after smelling blood.
How do I use it?
A Norwegian saying jeg fikk blod på tann is really saying that they 'got really into something and couldn't stop until it was finished'.
Vi begynte å pusse opp badet, men så fikk vi blod på tann og bestemte oss for å pusse opp hele leiligheten. — 'We meant to redo the bathroom, but then we got carried away and decided to redo the whole apartment.'
Don't use it like this
Få blod på tann is not something you say if you cracked or splurged. A Norwegian would never say Jeg tok en liten bit potetgull, fikk blod på tann og spiste hele pakken — 'I had one small crisp, got my teeth into it and ate the whole bag'.
No, no. You can't use få blod på tann about losing control with salty snacks, because it's really not about excess.
In true Nordic spirit (!), få blod på tann expresses determination and (a slightly manic) drive. Like when you start cleaning your apartment, realise how filthy it is and keep on going until it's all bright and shiny.
(Like that ever happened. But you get the point.)