a "japan flag" is the stain created on white or light coloured fabric, often on bed-sheets or underwear, when a total bottom has been railed bareback up the ass by a footlong and bleeds from their butt profusely
"That dude fucked me so hard last night that I woke up and my bed looked like a Japan Flag!"

"I had cheeky bum sex in the janitor's closet with the Quarterback, and he want in raw and rough, so now my ass is bleeding and my underwear have a total Japan Flag situation going on"
by n1ck_sP3ncerr July 13, 2016
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