The coco-de-mer is a kind of fan-leaf palm tree found only on two of the Seychelles islands, one of which is the island of Praslin, in the Indian Ocean. The coconuts produced by this tree are double-lobed and reportedly look like a coconut version of Siamese twins, or alternately like a part of the female anatomy. For this reason, the plant is also sometimes referred to as the double coconut. The strange shells were apparently found floating in the ocean long before anyone in the west knew where they were coming from, and their strange shape and mysterious origin prompted sailors to attribute them magical properties. More recently, the suggestive shape has inspired tales of its aphrodisiacal properties, especially if one cracks open the nut and eats the tasty, jelly-like interior. The nut takes up to ten years to mature and weighs fifty pounds when it's done. The trees don't actually begin to produce them, however, until the tree itself is about 100 years old. Production is consequently fairly low, and the nuts are a highly sought-after commodity.
Sources: http://www.foodreference.com/html/fcocodemer.html, http://www.airseychelles.co.uk/guide/praslin.html, http://www.santesson.com/aphrodis/fruitnut.htm