Common Phrase Meanings > D-Letter Sayings
(1) A term used in boxing; it refers to a boxer who's been knocked out or knocked down and is unable to continue fighting. (2) Someone who is incapable of performing normal functions due to reasons, such as serious injury or sickness. Example: a person who has broken their foot in an accident is unable to walk or run, thus it can be said they are 'down and out.'
This phrase is believed to come from boxing.

Boxing has multiple timed rounds that involve two fighters attempting to knock the other out. When fists are colliding with faces, the chance of being hit unconscious or being incapacitated for a short time is rather high. When a boxer does receive a heavy blow to the head, they either fall down and are unable to continue the fight, or they are knocked out completely, being unconscious. When either of those happen, the boxer can be described as being down and out.

This term has been used in boxing since at least the late 19th century. For instance, The Referee newspaper from 1899 reads: 

    "A short right arm swing on the neck administered by Rufe Turner sent 'Cocker' Tweedie, of Australia, 
     down and out in the eigth round of what was to have been a twenty-round fight at the Avon Theatre, 
     under the auspices of the Stockton Atheltic Association this evening."
* Tyler wants to go to work tomorrow, but he won't be able to for the next few days because he's down and out in bed with a serious illness.
The meaning of the phrase Down for the Count!
Origins pic for this idiom.

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Note: Tracking the origins of phrases is impossible most of the time! Mostly, what you'll find are the popular theories as to how an idiom originated, but it may or may not be necessarily so. 

In addition, quotes containing a particular phrase are usually taken from old newspapers, poems, or books that were written centuries ago, but this does not mean that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is thus from an older time. With that said, old quotes give some indication on how old a saying is.
Title image for the phrase Down and Out.