high-strung

This term evokes the image of a musical instrument string tightened to its absolute limit, where any further tension or a slight touch causes a sharp, sudden reaction. It describes a state of chronic psychological tension rather than a temporary mood, suggesting a personality trait characterized by fragility and hyper-reactivity.

While it can describe an anxious person, it is frequently applied to animals, particularly thoroughbred horses or nervous dogs, to denote an innate temperament that is volatile and easily spooked. It carries a more clinical or descriptive tone than "jittery," implying a deeper, more ingrained sensitivity.