back up
Preserved but no longer current.
His back is up, i.e. he is offended or angry; an expression or idea taken from a cat; that animal, when angry, always raising its back. An allusion also sometimes used to jeer a crooked man; as, So, Sir, I see somebody has offended you, for your back is up.
Era: ca. 1811 (Georgian)
Sources & notes
Historical definition reproduced from Grose’s Dictionary of the Vulgar Tongue (1811 edition, public domain; Project Gutenberg #5402, retrieved 2026-07-31). Reflects the attitudes of its era.
Common questions
What does “back up” mean?
His back is up, i.e. he is offended or angry; an expression or idea taken from a cat; that animal, when angry, always raising its back. An allusion also sometimes used to jeer a crooked man; as, So, Sir, I see somebody has offended you, for your back is up.
Is “back up” formal or informal?
No register label has been reviewed yet — treat it as neutral and check the usage notes above.