A beanis is what you can call a mexican/spanish penis, its misshapen and curved but not in a good way, and it looks like a redish kidney bean. Its the penis of a beaner, hence the name BEANIS.
(Guys are arguing over sumthing)
"Dude, stfu u godam Guido! i aint your fucking mother, clean ur own shit up!"
"well fuck u! at least i dont got a disgusting beanis!"