The Polar Plunge (aka polar bear plunge) is the act of getting up on new year's day (hung over most commonly) to jump in a lake. This act is best performed in Milwaukeejumping into Lake Michigan. Once taking said plunge, you become a member of the polar bear plunge club for life, much like the mile high club
Guy: hey, wanna take the Polar Plunge with me this new years?
Friend: umm, im kinda scared my dick will recede into my body from the cold
Guy: come on man, its worth it, you become a lifetime member once you do it.