improvise

/ˈɪmpɹəvaɪz/

IPA: /IH1 M P R AH0 V AY0 Z/

verb
  1. 1

    To make something up or invent it as one goes on; to proceed guided only by imagination, instinct, and guesswork rather than by a careful plan.

    He had no speech prepared, so he improvised.

Source: https://en.wiktionary.org/wiki/improvise