What does Masjävlar mean?

The word "masjävlar" is a derogatory term for Dalecarlians, who are usually known as masar.

https://en.wikipedia.org/wiki/Dalecarlians_(film)

I think this may be the answer:

DIRECTORS STATEMENT

... That they'll be a little nicer to each other, especially in family settings, since there's a lot of pressure in those relationships. But to begin with, I guess I wanted to tell the natives of the province of Dalecarlia a thing or two as well. Half of my family comes from Dalecarlia, from the region around Lake Siljan, and in my opinion, they're real characters up there. It's been fun raking them over the coals. At times I've been so sick of their behaviour, they're so incredibly full of themselves. I mean, they'll buy a summer cottage a half-hour drive from where they live just so they don't have to leave the province. Dalecarlia is Sweden's heartland, and everything about it is fantastic. When one of my relatives came to Stockholm, we went out for pizza – the delicious gourmet stone baked kind – and when we had finished our meal, this person looked at me and said: "Well, our pizza place back home in Rättvik certainly knows how to make a darn good pizza!" My intention was also to capture the special brand of humour they have in Dalecarlia, it's simultaneously warm-hearted and slightly brutal. If it isn't served up with a whole lot of love, it just sounds unkind. It's like irony – when someone would say "nice sweater" to you in school without really meaning it. That kind of humour wears you down, it's corrosive. At the same time, the film is a declaration of love to the people of Dalecarlia. You know, I picked up stakes and moved from Stockholm to Dalecarlia myself. These days I live in Falun and dream of being called a "kulla" (the Swedish word for a female native of Dalecarlia).

Source: https://www.europeanfilmawards.eu/en_EN/film/dalecarlians.5978