Someone, male, who enjoys anal sex to the point that they are constantly seeking it, providing loud vocal sounds during the actual act.
Those who have a constant need for anal sex, who are always available for anal sex. Similar to the concept of being a 'pig bottom' in that Nellie Bottom's will take any size penis, at any time, and show their vocal appreciation with more words, than just sounds of enjoyment.
Noteworthy: In some cultures, a 'nelly / nellie' bottom is more effeminate than others. Some assume that this is due to the loud noises they make, showing their enjoyment of receiving anal sex. It can be used in a 'derogatory' manner.