The phrase 'not happy Jan' was popularised by an Australian Telstra ad realised in 2002. It quickly entered the Aussie vernacular.
It is used when someone is pissed off at another person (who doesn't necessarily need to be called Jan) for stuffing up in a stupid way and has inconvenienced you - but is used when more annoyed than actually angry.
Argh! I can't believe that the postman hasn't delivered my package today! NOT HAPPYJAN!
This sentence is derived from the Swedish to English saying, "I'm angry with you." Except, the person who uses this is generally idiotic and/or foreign.
You killed the biggest, now more biggers are coming! Someone is not happy on you...
What you say to someone when you want to express that they have pissed you off because they have done, or forgotten to do, something. Consequently, you feel that they are a numbnut, idiot, dumbass, blonde, stupid etc.
To the random smelly sweaty guy who doesn’t shower nor wear deodorant and sits right next to you on the train – “NOT HAPPY, JAN!”
Greeting for couples who survived another month full of unprotected intercourses and have reached a menstral cycle, considering they have achieved this in the past.
Girl: “Babe, we have survived another month of porking without condoms!”
Guy: “Happy Not-A-Parents’ Day my love!”