Emotionally tense or excited, ready for action; or designed with specific keys or patterns, like a lock or a musical keyboard.
From the noun 'key,' which comes from Old English 'cæg,' possibly from Proto-Germanic 'gai-' (rod, wooden peg). The emotional sense 'keyed up' developed in the 20th century as slang.
The phrase 'keyed up' is clever—just like a physical key needs to be in the right position and tension to work, your body gets into a tense, ready state when you're anxious or excited.
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