I like to call this the dictionary game.
First, I start out with a random word and define it.
Then, the next poster picks the most complicated word in the previous word's definition and defines it... and it takes off!
Stirrup: the piece of tack on any saddle that holds the rider's foot in the correct position..
Tack: a wide variety of equipment used on horses (e.g. Saddle, bridle, girth, etc.)
And we begin...
MARTINGALE: A piece of tack, wrapping around a horse's neck, and connecting to the noseband of the bridle and the girth, usually used to keep a horse's head steady or held down.