What's "knock flat out" mean?

Native speaker’s answer
Rebecca
This is a combination of phrases! "Knock out" and "flat out." To "knock [someone] out" means to make a person unconscious, usually by hitting them or if they become unconscious due to a substance. "Flat out" means completely or as much as possible. So, to "knock flat out" means to make someone completely unconscious by hitting them. Ex: I worked flat out for the last two weeks. I need a break. Ex: I knocked out last night. Can't even remember what I did. => deep sleep Ex: She knocked him flat out in the boxing ring.