If you're skittish, you're unpredictable and excitable. You've probably seen skittish horses in parades — the loud noises and crowds of people make them very nervous and jumpy.
The adjective skittish isn't just for animals — humans can be skittish too. If you're sleeping in a haunted house, for example, you're likely to be skittish, jumping at every moving shadow. Skittish can also mean flighty in thought — like your boss who's skittish about giving you a raise and avoids discussing it. The word is thought to have come from the Scandinavian word skyt, meaning "very lively, frivolous."