1. From Judæo-German word for "Christmas"; derived from the medieval Latin "Natale Domini" meaning, "The birth of our God" or from the Latin "Dies Natalis", and French "Noël"
2. From the Hebrew "natal", meaning "to have been hanged".
3. From the Hebrew for "being taken away".
4. An acronym for "Nolad Yeshu Tet L'tevet", meaning, "J. was born on the ninth of Tevet."
The latter interpretations support a theory that J. was killed on Christmas. It may also be a derogatory nickname that was used to refer to J.
We don't learn on nittel nacht (Dec. 24 at night, X-mas eve) in order not to increase vitality [to kelipa].