Slang reference for the male genitals, more specifically for the scrotum and testes. Most effectively used in a more derogatory sense, than complimentary, though its use does not always, or generally, imply a negative note.
Has been used to describe a turn off, in a sexual connotation.
"I saw his nasty bits and left", is one such term, implying that the visual appearance was less than arousing.
While "his nasty bits were swaying side to side" wouldn't be considered derogatory, or mean spirited.