I have always been curious about word origins and this one is a real hoot. According to the book, Wicked Words, jinx has had an interesting transformation.
Jinx is a person or thing believed to bring bad luck; as a verb, to change fate for the worse. However, the word is an Americanism and surprisingly new, dated so far only to 1911. It is apparently a misspelling of the word jynx, which is the name of an Old World bird, also called the wryneck (from the way it writhes its head and neck when disturbed). A member of the woodpecker family, the jynx was thought to have magical powers. Witches used its feathers in making love philtres and other potions, with the result that by the sixteenth century the bird's name was synonymous with "charm" or "spell." Thus, it seems, the jynx became a jinx.
What a great word to work into a novel, either as the bird, or a magical potion.