A variation of a "Wet Willy" but where emphasis in the "Wet Willy" is placed upon saliva, the emphasis in the Steamboat variation is placed upon ass sweat.
"Hey Brian, don't Steamboat Willy me while I tie my shoe."
To burrow your nose into an anus durring sexaul playtime. Also known as a Grundle.
The key to any good Grundle or steamboat willy is copious amounts of ass sweat.