Something dishonorable is embarrassing or shameful. If you get a dishonorable report card, it probably means your grades could've been better.
The adjective dishonorable often describes a soldier's discharge from the military — a dishonorable discharge means the enlisted member has been found guilty of a crime by a court martial. Its opposite is an honorable discharge, when a solider in good standing leaves the military. You act with dishonorable behavior when you bully someone or act cruelly. Honorable comes from the same Latin root as honor, honorem, "dignity or reputation," and dis- here means "lack of."