out like a light
out like a light {adj. phr.}, {informal}
1. Fast asleep; to sleep very quickly.
Tom got so much fresh air and exercise that he went out like a light as soon as he lay down.
As soon as the lights were turned off, Johnny was out like a light.
2. In a faint; unconscious.
Johnny was hit by a ball and went out like a light.
After she read that Tom had married another girl Jean was out like a light for several minutes.
Compare: OUT COLD.
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Source: A Dictionary of American Idioms