Pessimize

Pessimize combines pessim- (the root of words like pessimist and pessimistic) with -ize to create optimize's opposite. So, just as optimize means "to make better or more efficient," pessimize means "to make worse or less efficient."

Because most people don't seek to pessimize things, you're most likely to see pessimize used as a diss, after someone accidentally makes something worse. For example, if a software update makes users' iPhones run slower, you might see a user post "Wow, this new update has really pessimized the iPhone" to social media.