Definition of fuck boy:
(n) :
(slang, vulgar) A young man who sleeps with partners without any intention of having a relationship with them.
(n) :
(slang, vulgar, derogatory) A weak and contemptible man.
(n) :
(prison slang, vulgar) A prisoner who provides sexual favors to a stronger inmate, either as the result of rape or in return for protection and gifts.