Is this a saying in Norwegian?
Yes, it is
What does it mean? like, loudly and or unclearly?
A waterfall is (hopefully) a constant stream water. So when someone is talking like a waterfall, it will never stop (unfortunately). I thought this was a very common saying. In Dutch we made one word of it: "spraakwaterval"
Foss remains in some northern English dialects and as a place name - High Force, Falling Foss, Thomason Foss.
We have the same saying in German. ☺
In English there is something similar: she's like a babbling brook.
In certain parts of America we would say "she is a motor mouth"
In Romanian language we say: "Hun snakker som en knust mølle" (she speaks like a broken mill). Because sometimes a broken mill cannot be stopped.
Like, she runs at the mouth?
Ironically, I learned about norsklærer Karense from the comments on these lessons and I first learned this idiom from her. I love the idiom, and I used to have a best friend named Sam Foss.
like this http://www.europeanwaterfalls.com/wp-content/uploads/Kjosfossen-7-G.jpg like my mom
Love foreign idioms.
Does this mean she talked a lot, or quickly?
A lot, and without stop.
Did you tell her to holdt kjeft? XD I think that's the term.
Verbal diarrhoea !
This little puddle of consciousness, treading water...
Or more commonly " verbal diarrhoea "