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Phrases and Idioms - Meanings and Origins of Phrases | If you're interested, check out this animal phrases list, it has dogs, cats, horses and more!
Losing money. Being in debt.
Note: The origins for most phrases and sayings cannot be said with a certainty. What's provided are theories that may be plausible to how a phrase originated, but not necessarily so.

In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms 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 from an older time.
* James created a furniture company knowing ahead of time that he'd be working in the red, but he hopes business will pick up in the following months so he can start turning a profit.
Know Your Phrase - The Origins And Meanings For Idioms And Phrases > I-Letter Sayings
The meaning of this common expression.
The origins of this saying.
It's believed this phrase originates from accountants who are selective on which color ink
they use when making entries in a firm's book. If someone is doing business at a loss,
instead of using black ink, accountants will use red to write down the deficit. Indeed, the
color red is prominently used in businesses to signify a loss, while black is used to indicate
a profit. Thus, a company that's interested in making money would desire to be 'in the black'
rather than be 'in the red.'

This idiom seemingly originates from the early 1900s. There are numerous newspapers from that period of time which use the phrase when referring to businesses functioning at a loss. For example, the Daily Globe from 1927, under the subheading 'Failed to get Interest,' reads:

"I regret to report to you that the people who put money into it failed by $30,000 to earn even
operating expense and depreciation. The comany is in the red all the time."