Heh, And all these years I thought it was what the barefoot Captain did in the wee hours after the Rum Shack closed.
broadly speaking - it's when the beam at the waterline is greater than the beam at the sheer - see the stern of the venerable Hinckley 'Picnic Boat' for an excellent example!
Tumblehome also refers to the gentle slope inward between the bottom of an exterior bulkhead to the top on all decks not only the hull. This makes passageways more generous by being wider where the shoulders are and narrower at the feet. Furthermore, it gives the vessel a more aesthetic line preventing the "homemade" slab-sided appearance of straight, vertical bulkheads. On the interior of the same bulkheads with tumblehome, those designers who are actually decorators and not really proper yacht interior designers are driven nuts by an inwardly slanting bulkhead that require window and door frames to be wider at the bottom and narrower at the top in order to be square on the inside. Also makes them crazy hanging drapes.
Tumbledown Hey, I once anchored at Tumbledown Dick Head in Maine. No kidding!! http://www.us-places.com/map-places.php?placeid=577396