co·co-de-mer 
(kōkō-də-mâr)
(kōkō-də-mâr)n.
1. A fan-leaved palm (Lodoicea maldivica) native to the Seychelles, having an extremely large seed enclosed in a hard shell resembling a pair of coconuts.
2. The seed of this plant. In both senses also called double coconut.
[French : coco, coconut + de, of + mer, sea.]