Bucket of water
Any orifice (other than a mouth) that, from overuse, has stretched well beyond the circumference of the largest of penises, making anyone engaging in penetration with it unable to feel the walls on either side. It may even splash.
Example:
Guy 1: I heard Rico is into fisting now.
Guy 2: I know! I hit that last week and it was like fucking a bucket of water!
Guy 1: I heard Rico is into fisting now.
Guy 2: I know! I hit that last week and it was like fucking a bucket of water!