1
general::
Also, out for the count;out like a light. Unconscious; also, asleep. For exam ple, He crashed into the wall and was out cold, or Willie punched him too hard, and he was out for the count or Don't call Jane; she's out like a light by ten every night. The adjective cold refers to the lack of heat in a dead body and has been used to mean "unconscious" since the second half of the 1800s. The first variant comes from boxing, where a fighter who is knocked down must get up before the referee counts to ten or be declared defeated; it dates from about 1930. The last variant alludes to turning out a light and dates from the first half of the 1900s.
American Heritage Idioms
2
general::
Phrase(s): out cold [and] out like a light
1. Fig. unconscious. • I fell and hit my head. I was out cold for about a minute. • Tom fainted! He’s out like a light! 2. Fig. intoxicated. • Four beers and he was out cold. • He sat in his chair at the table, out cold. 3. Fig. sound asleep. • After a few minutes of tossing and turning, she was out like a light.
McGrawhill's American Idioms And Phrasal Verbs