patronizing
Acting overly nice while treating someone like they are dumb.
Patronizing means treating someone as if they're less intelligent or capable than they really are, often while pretending to be helpful or kind. When someone speaks to you in a patronizing way, they might use an overly sweet voice, explain obvious things you already understand, or act like you couldn't possibly figure something out on your own.
Imagine a classmate explaining how to tie shoes to a fifth grader, or a babysitter saying “Good job using your fork!” to a ten-year-old. That exaggerated praise for simple tasks feels insulting because it suggests you're much younger or less competent than you actually are. A patronizing comment might sound nice on the surface, but underneath there's a suggestion that you need extra help with basic things.
Adults sometimes patronize children by oversimplifying everything or refusing to give real answers to genuine questions. Teachers can be patronizing when they praise students for work that's far below what those students are actually capable of.
The opposite of being patronizing is treating people with genuine respect, giving them credit for what they know, and explaining things at the right level for their actual abilities.