Bunk-balls refers to a situation where two men are having sex with one woman, double penetration style where the woman is riding one gentlemen as he is lying or perhaps sitting, and the other gentlemen is penetrating her while he is behind both of them. Variations are acceptable as long as it creates a situation where the bottom man's scrotum lies beneath the top man's scrotum, stacking them vertically like bunk beds. Hence the term, "bunk-balls".
Steve: "Tyler, see that chick over there?"
Tyler: "Yeah, the hot one?"
Steve: "Yeah, the one with the bubble booty giving me and you the "I need DP-face".
Tyler: "Yeah, I see her. Damn she is smokin hot."
Steve: "Yep, I got four words: me, you, her, bunk-balls."
Tyler: "Dood, that's five words."
Steve: "Well, the bunk-balls is hyphenated."
Tyler: "Oh, I see, but does that make it one word?"
Steve: "Dood, shut up, who cares, go get her number so we can rub shafts later."
Tyler: "Sweet, ok."