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.