One of the most accurate barometers of a cat’s mood is its tail. When held high, it communicates confidence. Curled around another cat’s tail or human legs, it signals friendliness. And tucked below or between the legs, it signals insecurity or anxiousness. The upright bottle-brush tail is an unequivocal sign that your cat is feeling threatened. And when combined with the arched back, upright hair along the spine, and unsheathed claws, the stance is a dead giveaway that you should back off, Sueda says. Source: WebMD