Today's word is not one to use in polite company: ett knull means 'an act of sexual intercourse'. The one-word English translation would be 'fuck', except it's probably even more vulgar (knull is never used in any other context, like 'fuck' can be used as an expression of frustration, surprise, or an intensifier).
It's been used in Swedish for centuries, and comes from the verb knulla.
As we all know, Swedish speakers love compound words, and yes, knull appears in plenty: knullkompis (fuck buddy), knullbar (fuckable), knullrufs (bed hair).
Well, when Marvel Comics launched an ad campaign to tease a new villain arriving to their universe this year, the slogan was “knull is coming” (that's the name of the character). Needless to say, they probably didn't get the reaction they intended from their Swedish fanbase.
But we know what you're really interested in – the linguistic history of knull, of course!
You can also use knull to talk about a person you've had sex with, although it's quite derogatory for obvious reasons.
But it didn't always have a sexual connotation, since it originally meant 'to bump' or 'to push'. It comes from an Old Germanic word, and is related to the verb knüllen in today's German, which might make Swedish speakers giggle but actually means “to crumple”.
One of the earliest recorded uses of Swedish knull was in, believe it or not, a bible from the 1500s. But it was used as an onomatopoetic verb describing the sound a pigeon makes.
Slowly it entered the Swedish vocabulary as a euphemistic term for sex, and during the 1600s it wasn't seen as vulgar, but can be found in old love poetry. These days, we'd suggest you avoid that unless you really, really know your audience.