Per the BBC, the United Kingdom discussed implementing a 'circuit breaker' in October. Israel, New Zealand, and Singapore have used 'circuit breakers', and it seems that Singapore was the first to use the term. Are there any earlier instances of 'circuit breaker' used in an epidemiological context?
Answering at face value, the term "circuit breaker" comes from electrical engineering, where it's a device that protects a circuit (say, the wiring in your house) from overload. Most commonly, if there's a short circuit causing electricity to flow in unintended ways, resulting in a surge of current, the circuit breaker kicks in to physically break the circuit, stopping the current from going so high that overloaded wires start melting or catching fire.
Extending the analogy to infectious disease, "current" is the rate of increase of infections, and the "circuit" is the people that represent the potential hosts of the disease. So if infections start spinning out of control and you risk hospitals getting overloaded, you can apply a "circuit breaker" (lockdown) to forcibly separate out people, stopping the overload. And once things are back under control, you can "reboot" the system by turning off the circuit breaker, and everything is back to what it's supposed to be.
The obvious flaw in the analogy (at least to us electrical engineers) is that if you turn the circuit back on again without diagnosing and fixing the actual issue, you'll sooner or later be back in overload again. Of course, this is exactly what happened, so maybe it's not a flaw after all.
While I believe Singapore was indeed the first country to use the term for a national COVID-19 lockdown, the medical analogy predates COVID: here's a 2006 Nature paper about using endocannabinoids as a "synaptic circuit breaker", and a 2016 paper about adolescent education as a "circuit breaker" for the "non-communicable disease epidemic" (obesity, diabetes, etc).