The hindmost of something is the part that's farthest back, like the caboose of a train, the tail of a plane, or an elephant's great big...rear end.
Breaking down the meaning of hindmost is a pretty simple affair, since hind means "rear" (a dog's hind legs are its back legs) and most means, well, "most." It should also come as no surprise that handy synonyms for hindmost include backmost, rearmost, and endmost. The phrase "devil take the hindmost" refers to a self-serving attitude without regard for others.
