Feeling nervous, tense, or irritable; in a state of anxiety or anticipation that makes one easily startled or upset.
This phrase dates to the early 20th century and uses the metaphor of standing on the edge of a cliff or precipice, where one must maintain perfect balance and alertness. The 'edge' represents the precarious mental state between calm and panic, where any small disturbance could cause one to 'fall' into emotional distress.
The phrase perfectly captures the physical sensation of anxiety - that feeling of being poised on the brink of something unpleasant. Neuroscientists have found that anxiety literally puts our nervous system 'on edge,' heightening our startle response and making us hypersensitive to stimuli, just as if we were balancing on an actual precipice.
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