disimprove

dis- +‎ improve

disimprove (third-person singular simple present disimproves, present participle disimproving, simple past and past participle disimproved)

1. (transitive, rare) To make worse.
   • 1642, Jeremy Taylor, The Sacred Order and Offices of Episcopacy or Episcopacy Asserted against the Arians and Acephali New and Old:

     To which may be added this advantage, that the services of churchmen are rewardable upon the church's stock: no need to disimprove the royal banks to pay thanks to bishops.

2. (intransitive, rare) To deteriorate.
3. (informal, rare, influenced by the German verb verschlimmbessern) To try to make something better but end up making it worse.

• improvised