In the red means owing money, or overdrawn. Slip into the red, still in the red, deep in the red.
It comes from bookkeeping, where losses were entered in red ink so they could not be missed. The opposite is in the black, and the pair is one of the few pieces of accounting that everybody understands.
1phrasein the redOwing money; overdrawn.
“I'm in the red again this month, so I can't afford to go out.”
in the red is a state.
From the accounting practice of entering losses in red ink.
From the standard accounting practice of using red ink to denote negative values, especially a net loss. The first known written use of the phrase is from the "Wise-crack dictionary" (1926) by George H. Maines and Bruce Grant.[http://www.phrases.org.uk/meanings/202000.html]
Etymology from Wiktionary, CC BY-SA
Practised in Shopping & Money