Is lunsj a norwegianised(?) lunch or did we steal it? :)
It is a word taken from English into Norwegian. Another word for lunch is "formiddagsmat" wich we used in my childhood some decades ago.
It comes from English. Like a lot of foreign words in Norwegian the spelling was adapted to make it fit better.
From English lunch, according to Wiktionary.
When answering "She's lunching" I got "She's lunch". Weird Norwegians....