During anal sex between two homosexual males, the testicles of the partner engaging as "bottom" will serve as protective boundaries in regards to the excessive fecal splashings that occur during such interludes.

a vagina with a ball sack underneath, but the sack has no balls in it

Verb. flicking the sphinkter anus of the girl you are having sex with.
While screwing doggy-style, flick the girls unsuspecting "brown eye" to send a shock-wave through her body. This causes her vagina to tighten and increases pleasure for the guy. The girl will experience some discomfort.
Warning: performing the mudflap will result in future trust issues!!

Used to describe a girl with a very flat/unattractive ass

In contrast to pissflaps, which refer to the labia minora, mudflaps refer to the peculiar instance of anal warts in which the growths form a densley-populated forest of protrusions that obscure the hotpocket itself.

a mudflap is where your girlfriend shits on your stomach and then drags her vagina through it.

Homosexuial's Foreskin