keyed up

Example: “The night before the exam, he was too keyed up to sleep.”

Origin: American slang from 1889, describing someone excited or high-strung, and several mechanical images compete for the underlying picture. One points to winding a key-operated mechanism — a clock, a music box, a wind-up toy — to its maximum tension, primed and ready to spring into action. A second, closely related sense reaches for tuning an instrument's strings by turning its keys or pegs to raise their pitch, extended figuratively to raising a person's emotional state the same way. A third, more general engineering sense of "keying up" means tightening a part firmly into place under real mechanical tension. A less-supported fourth theory reaches for telegraph operators, whose rapid, alert use of a transmission key supposedly lent the phrase its sense of heightened readiness. None of the four is confirmed as the definitive source, though all share the same core image of something wound, tightened, or tuned to a state of readiness just short of release.