The term "board off" doesn't have any official antonyms, as it's not a widely recognized phrase. However, if we break it down, "board" can refer to a few different things: a flat piece of wood, a group of people who manage an organization or company, or an action of getting onto a plane, train, or boat. In that case, some possible antonyms could be "unboard," "disembark," "disperse," "dissolve," or "off-load," depending on the specific context. Without knowing the exact meaning and usage of "board off," it's difficult to suggest precise antonyms.