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.