What does it mean?

Tenke koffert – ‘think suitcase’ – is a slang expression that means ‘associating something with sex’, but without that ‘something’ necessarily having anything to do with sex at all.

It's similar to the English expression ‘having one’s mind in the gutter’.

Basically, by saying that someone tenker koffert you are accusing them of having a very dirty mind.

Origins

Although it’s not a new expression, it’s pretty informal and mostly used by young Norwegians. (At least it should be. All you men-over-50 using it, please stop. You're being creepy.)

The Norwegian language guardian Språkrådet does not know exactly when or where the expression originated, but writes that it has at least existed since the 1950s. 

It's a Freudian symbol referring to something you can ‘put stuff into’ (i.e. the female genitals, for those needing to få det inn med t-skje — have it spoon-fed.)

How do I use it?

Slutt å tenke koffert! — Get your mind out of the gutter!

Tenker du litt koffert, eller? — Having dirty thoughts, are we?

Other options?

Don't worry if you're not comfortable with saying tenke koffert. Honestly, it's not our greatest expression, but it is good to know what it means in case it comes up.

Other ways of saying that someone tenker koffert are:

Tenke snuskete — an expression that older people probably would be more comfortable with (at least the under-30s would be more comfortable when hearing them say it).

Tenke skittent — thinking dirtily, meaning 'having a dirty mind'

Tenke dirty — also thinking dirtily. Yes, I'm afraid to say that we've appropriated the English term and made it our own. While you wouldn't get away with using it in an exam (you'd need to use the Norwegian term), no one would question you for using it in an informal, oral setting. (Oral as in verbal. Not the other kind, slutt å tenke koffert!)