Origins of "Ness"
The origin of the name of the river Ness is not very clear. The Gaelic for Inverness is Inbhir nis, Inbhir or "Inver" means mouth (as in river mouth not the mouth on your face) so Inbhir nis means "mouth of the nis".
The Gaelic word nis means "now", however the old Norse word nes means "headland" as does the anglo-saxon word naes. There are many place names in the north of Scotland ending in ness, nis or nish which come from old Norse but it seems that this is generally not thought to be the case for Loch Ness, Inverness and the River Ness.
There is a folk tale which even in the capacity of a myth gives a strong case for the Gaelic meaning of Ness. This story is recounted by J. F. Campbell in Popular Tales of the West Highlands on page 146, told to him sometime before 1860 by a Mr. Thomas MacDonald, gamekeeper at Dunrobin.
"Where Loch Ness now is, there was long ago a fine glen. A woman went one day to the well to fetch water, and she found the spring flowing so fast that she got frightened, and left her pitcher and ran for her life; she never stopped till she got to the top of a high hill; and when she was there, she turned about and saw the glen filled with water. Not a house or field was to be seen! "Aha!" said she, "Tha Loch ann a nis." (Ha Loch an a neesh). There is a lake in it now; and so the lake was called Loch Ness (neesh)."1
J. F. Campbell also comments on the previous page of this book that the name might mean "The weasel lake", "The lake of the falls" or "The lake of the island".
1. CAMPBELL, J. F., 1983. Popular Tales of the West Highlands, Volume 2. 2nd Edition. Hounslow: Wildwood House Ltd.