also: ふりちん
noun
fully exposed penis; going commando (male)
Colloquial, somewhat vulgar term for a male whose penis is completely exposed, typically because he is not wearing pants or underwear. Often used in casual or humorous contexts.
彼はパンツを忘れてフルチンで歩いていた。
He forgot his underwear and was walking around with his penis fully exposed.
子供の頃、よくフルチンで川遊びをした。
When I was a kid, I often played in the river completely naked (with my penis exposed).
Likely a contraction of 振る (furu, 'to swing') and ちんこ (chinko, 'penis'), referring to the penis swinging freely when exposed. The variant フリチン may be influenced by 振り (furi).