A polar bear, when referring to a person, is a member of "the Polar Bears", a society which advocated swimming in the icy ocean in the winter. This was supposed to give you a rigorous constitution, and a strong character to boot. Though I have never swam for any length of time in the winter ocean, I have dunked in it briefly and I can understand the appeal: the sudden assault of cold viscerally terrifies your body, and seems to cause it to dump massive ammounts of adrenaline into your bloodstream (or something). The overall feeling once you emerge from the water is tremendous tingling on your skin, very rapid heartbeat, and a giddy, insane euphoria.