Sissonne is one of those unique ballet terms that doesn't originate in the movement itself, and yet it is one of the earliest recorded terms. Most commonly attributed to le François César de Roucy, le Comte de Sissonne, sissonne was likely named for its creator and for the simple fact that he excelled at this step.
The dancers in the court of King Louis XIV were not professional dancers as we think of them today. All noblemen were trained in dance and comportment, and those who happened to be at Versailles often danced in the ballets put on at the court. Le Comte de Sissonne was a page de la grande écurie at the court of Louis XIV so it is highly likely that he participated in the performances and the training.
Sissonne is pronounced [sisɔn]. Si- is pronounced similarly to "see" in English. -ssonne- sounds like “sun” but the u sound is a cross between the u in “sun” and the o in “on.” The n is pronounced at the end of the word.
A sissonne is performed by jumping off of two feet and landing on one. It can travel in any direction and can finished with the second leg lifted or closing into a crossed position (3rd or 5th). Sissonne can also be performed as a chassé relevé (usually in pointe work). Both forms are darting movements.
NOTE: I’ve often seen sissonne described as or confused with scissors. (Scissors are ciseaux in French). I’ve also read in ballet posts that sissonne means split but I can find no evidence of this. If scissors present good imagery for your dancers, use the idea, but be sure to reinforce that this step is named for a person, rather than for the object.