unawares

/ˌən-ə-ˈwerz/

un- + aware + -s, adverb suffix, from Middle English, from -s, genitive singular ending of nouns

adverb

  1. without design, attention, preparation, or premeditation

  2. without warning : suddenly, unexpectedly

idiom

  1. to affect (someone) without warning or without being expected

The rainstorm caught us unawares.

idiom

  1. to surprise (someone) by something unexpected

She was taken unawares by the sudden change in plans.