The untanned spot left on your inner thigh from where you testicles (aka ball sack, balls, sac, bag etc...) were lying when tanning or nude sun bathing.
"I kept moving my balls around while tanning to prevent sack shadow."
"I hate when I see him in briefs, his sack shadow is obvious."
"I hate when I see him in briefs, his sack shadow is obvious."
by muckwa July 7, 2012
Get the Sack Shadowmug.