To disavow is to deny support for someone or something. You might feel dissed if your biggest donor decides to suddenly disavow you in your run for president.
Disavow comes from a combination of the Old French prefix des- meaning "opposite of" and the word avoer meaning to "acknowledge, accept, recognize." When you disavow, you are doing the opposite of acknowledging or accepting. You're rejecting or denying. When you disavow something you've done in the past, you reject or deny what you've done.