In fighting games, a hitbox is the area that a given character's attack covers. Say for example, the length of a character's arm as they extend their fist out for a Light Punch.
When these hitboxes are insanely huge or irrationally out of range, these are known as niggahitboxes.
(talking about Marvel Vs Capcom 3)
P1: Damn I hate Wolverine's Dive Kick!
P2: Yeah, that shit got a niggahitbox!