A man in the armed services that participates in a blanket party, which is the beating of another soldier at night when everybody is sleeping. this usually happens in boot camp if a soldier does something wrong that effects the whole platoon or unit.
Bill: do you know what a skivvy ninja is?
Fred: Yeah, i was a skivvy ninja in boot camp, we beat the shit out of guys that kept screwing up.
Bill: wow, your a badass!