It is used to describe a particular person who has done something a bit dodgy, like nicking a fag end from an ashtray or just being generally trampy.
It's not as specific as Pikey, but has similar connotations.
It was also used in shouting competitions, where one person in the group would call somebody a Nubber and they would be answered with, "Did you just call me a NUBBER!" until it ended up whoever could scream it the loudest got mucho Kudos.
"Have you been wearing the same socks for 8 days? What a Nubber!"
nubber can also be used for small objects that the name is unknown for.
Ex:Hey betch hand me that little nubber over there.