'Fell' is an old word meaning 'terrible' or 'deadly'
Note
There are two 'Fell Winters' in Tolkien's works; this Fell Winter of the Third Age is not to be confused with the Fell Winter that occurred long before in the First Age, nor is it to be confused with the Long Winter of some 153 years earlier
The long harsh winter of the year III 2911, in which Eriador was invaded by White Wolves. The terrible floods of the following spring caused the destruction of Tharbad.