Used in the phrase 'taken aback', meaning surprised or shocked in a way that causes one to step back mentally or physically.
From 'a-' (on, at) plus 'back', originally a nautical term meaning 'backward against the mast' when sails were pressed by contrary winds. The figurative sense of being surprised developed by the 18th century.
This word survives almost exclusively in the phrase 'taken aback', preserving an ancient sailing metaphor where ships were literally pushed backward by unexpected winds. The physical sensation of being pushed back by surprise perfectly captures the mental experience of shock.
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