When a man's penis is visible through his clothing - defined specifically by a shadow of the glans (head) but also the shaft. Usually most visible when wearing grey sweatpants commando or with loose underwear. But can also be spotted when wearing just tighty whities or just tight boxers (too tight and everything gets squashed together and the shadow becomes a smooth bulge). Sometimes visible in swimwear but often there's too much bagginess going on to see any definition.

Why it's important: It's an arousing reminder of what's down there. Quite like when a woman's nipples are visible through her top.
When you double take an unattractive guy as he walks passed and you have to explain: "I don't actually think he's hot... he just had really good penis shadow."
by ghost face 88 August 2, 2014
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