To unnerve someone is to make them feel scared or confused. Haunted houses are meant to unnerve you — though there may be nothing that unnerves you more than the first day at a new school.

The original meaning of unnerve, dating back to the 17th century, was "to destroy the strength of." The source of this definition was a now-obsolete meaning of nerve, "to give strength and vigor." Today when you unnerve someone, instead of destroying their strength, you destroy their composure — in other words, you make them nervous or uncertain.

Definitions of unnerve
  1. verb
    disturb the composure of
    synonyms: enervate, faze, unsettle
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    cause to lose one's nerve
    type of:
    discomfit, discompose, disconcert, rattle, untune, upset
    cause to lose one's composure
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