Fuck Nigga (Fuck Boy) is a slang term used to describe someone that's perceived as being weak, not real, or just a pussy. This term is heavily used in hip hop music to describe rivals and people they believe are fakes. "Fuck Boy" is the exact same thing as a "Fuck Nigga," the only difference is the word boy is switched for nigga.
A snitch is classic example of a fuck nigga, a snitch will be all cool around their peers, but when pressured, they will tell on all of their friends without hesitation. Someone who's constantly picked on without retaliating can also be considered a fuck nigga. Someone who's constantlly pretending to be something they're not can also be called a fuck nigga.
Rapper Chief Keef has a song titled "Fuck Nigga" in which he raps "Pull up on your block like skrrrr - Fuck niggas gon' freeze up like deer." Chief Keef is basically saying they're not about the life they talk about, when a real nigga pulls up on them, they won't know what to do.