means A feeling of nervousness or anxious unease, often shown as trembling or fidgeting before something stressful.

from An American coinage from the early 20th century, built on "jitter," meaning to move in a quick, jerky, unsteady way. The word is imitative — its very sound mimics the rapid little twitches it names, a cousin of fidgety words like "jiggle" and "jerk." By the 1920s and '30s the "jitters" had become the nerves themselves, and the dance-floor energy of the age gave us the "jitterbug," all the same nervous bouncing turned into a beat.