red line - definition and synonyms

noun [countable] 

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/ˌred ˈlaɪn/
singularred line
pluralred lines
  1. a statement, typically by a government, of acts that they regard as unacceptable and will respond to

    The use of chemical weapons during war is a red line issue.

Word story
Red line has been used since the 1970s to denote a limit beyond which someone must not go without facing severe consequences. It suggests the idea of a physical boundary marking the limits between acceptable and unacceptable behaviour, and is related to expressions such as ’cross a line’ and ’draw a line in the sand’.