This is a term that you NEVER say to an ensign. The rank insignia for an ensign is a single gold colored bar. A senior enlisted person, E8 or E9, can get by using this term in the open. But anyone below that might get in trouble.

The term is used for a few different reasons. First, because new ensigns are a pain. Secondly, because new ensigns are a pain, and finally because new ensigns are a pain.
An Ensigns collar device is a single gold color bar. On the sleeves there is one gold band and on the shoulder boards there is one gold stripe. So as you can see, the poor ensign is stuck with his nickname.

There is an urban legend about a new Ensign walking down the sidewalk at Norfolk Naval Base, or any other base. He comes up to an old Master Chief who has been in the Navy longer than the Ensign has been alive. They pass and the Master Chief says hello sir, but does not salute. The Ensign takes offense to this and asks the Master Chief if he forgot something. The Master Chief says “Yes sir I did.” He reaches into his pants and hands the Ensign 50cents and tells him, “Go call you mom and tell her you met a real sailor today.”

Seriously it is not that bad with all new ensigns. Some of them are great and come right in and do their job with little or now guidance from the Chiefs under them. Others need to be guided around by their Chiefs and other enlisted guys. Truth be know, some new sailors are just as bad, but since they are enlisted we just call them NUB.