All the other definitions are absolutely wrong. The following is correct.
Purin /pu-REEN/ (n.): Testicle.
(You may have encountered the plural form of Purin, i.e. Los Purines pu-REE-nez
Technical definition--Purin (n.): A pink cluster of mass that runs, floats, sings, and sleeps in some bootleg Japanese game called Smash Bros. or something. Light and easily jacked out of the level. (This phenomenon is known as testicular ejaculation.) Has a special move in which it rotates its body into another opponent and Up + A's the him. This move is known as the Testicular Torsion Attack, or more fondly known as the Textbook move. A Purin should never be confused with its cancerous cousin known as Kirby.