forfend

English speakers have been using forfend with the meanings "to forbid" and "to prevent" since the late 14th century, and the meaning "to protect" since the late 16th century. These days, however, the "forbid" sense is considered archaic; we only use it (as in our second example) in phrases like "heaven forfend" or "God forfend." Forfend comes from for- (an old prefix meaning "so as to involve prohibition, exclusion, omission, failure, neglect, or refusal") and Middle English fenden (a shorter variant of defenden, meaning "to defend").