[Andy B]

The term comes from Faro, a card game that was popular in the latter 19th and early 20th century, but which has since died out more or less completely. The dealer dealt from a box or "case," and the last card of a particular rank was the "case" card. This term carried over into poker. Faro is also where the term dealer's "box" comes from, and probably other terms I'm not thinking of right now.