Swedish for "No".
Used when you are fed up of someone, no matter what they are doing to annoy you.
Saying "Nej" (Pronounced "Ney" or "Nay" depending on accents) will automatically make you right in any situation and anyone in the same room at the time of this word being announced will never annoy you for the rest of the day.
This seemingly magical theorum works every time unless the person(s) is a total douchebag(s), in which case they can be silenced by a simple broken neck.

When you eat dirt for the first time and leave your family to build a shack in the woods

Not Even Joking.
Used, mainly on social network sites, messenger programs and through text messages, when someone is very serious about what they are saying.
It is usually said in a situation, when one person is trying to assure another that they are, in fact, serious, not being sarcastic.

A blobbly like substance that will evolve and become a living organism, but only once every 56 years when a Frenchman approximately 29 years of age will make love to a fat flap while under a curse.
It will differ in gender but most known "Nej's" found appear to be female but know one has ventured deep enough to determine if it truly is a female or a cross-breed.
Has a known appetite for literally anything edible or inedible and will store food it its creases to hold out for the long winter ahead