Patronize is a verb or, as in this case, an adjective. Firstly, the word "patronize" is derived from the word "patron." A "patron" is a noun describing a person who provides some kind of support to a person, activity, or organization. It also is commonly used to describe a person who is a customer of an establishment that renders services or goods to the public. When we add the suffix "-ize" to a word the meaning changes to cause to become, treat in a particular way, or follow a specific practice. "Patronize" could mean to frequently buy services or goods from a particular establishment. However, as an adjective/verb, it means to speak to another person as if they were inferior or less than but in a seemingly kind tone so as to not reveal the rudeness of their behavior. Imagine how an adult or parent would speak to a child, the kind of voice they would use, then apply that same tone to another adult. That would be quite rude, right? So, you might here someone say, "Don't patronize me!" in response to that behavior.