From de- +‎ accelerate.

deaccelerate (third-person singular simple present deaccelerates, present participle deaccelerating, simple past and past participle deaccelerated)

1. (nonstandard) Synonym of decelerate
   • 2017, Shengqiang Yang, Surface Finishing Theory and New Technology, page 84:

     This impact deaccelerates the abrasive blocks and then immediately accelerates abrasive blocks in a rebound direction as the vibration reverses.