The phrase "be the back of" generally means to support or assist someone or something. The antonyms for this phrase include to reject, to disapprove, to oppose, to hinder, or to obstruct. In situations where someone is the back of a person or organization, they offer their help and assistance. However, if they oppose or reject, they are acting against the person or organization. Similarly, if they are obstructing or hindering, they are stopping the progress of whatever project or initiative that is being undertaken. Therefore, it is essential to understand the antonyms of "be the back of" to avoid any misunderstandings or miscommunications.