What does it mean?
Directly translated, pling i bollen means a ping in the bowl. Norwegian people use the onomatopoeic expression to say that someone is empty-headed or stupid – from the ‘ping’-like noise an empty bowl makes when you tap it.
Someone who is pling i bollen is crazy. Be it their reckless behaviour, their stupidity, their tactlessness – pling i bollen is a pretty harmless and non-aggressive way to tell someone that you find their action, statement or attitude completely bonkers.
How do I use it?
You combine pling i bollen with the verb å være ('to be') in order to say that 'X is' pling i bollen.
It's often used as a question:
“Er du helt pling i bollen, eller?” (‘Are you completely out of your mind?’)
Who uses it?
Pling i bollen an old Norwegian expression that is less commonly used by young people than by the older generations.
Young people might prefer er du helt dust, eller?
It means the same, but dust is a (very) common insult in Norway – especially among teens – that might best be translated into ‘fool’ (Google Translate suggests ‘jerk’, but that’s just one of the many meanings dust can have).
Another version is er du dum, eller?
The advantage of pling i bollen is that, unlike with dust and dum, the intensity of the insult is pretty low-risk. If you call a Norwegian dust or dum, chances are they get offended.
But if you call them pling i bollen, they'll probably find it funny as 1) it's an old expression they don't hear too often, which 2) makes it really impressive that as an immigrant you've picked it up.