also re-enforce, "add new force, strength, or weight to," c. 1600, originally in military sense, from re- "again" + inforce, variant of enforce "drive by physical force; fortify, strengthen" (compare re-enforce, and see en- (1)). Related: Reinforced; reinforcing.
The ordinary form (rein-) has been so far divorced from the simple verb (formerly inforce or enforce, now always the latter) that it seldom or never means to enforce again, as when a lapsed regulation is revived. For that sense re-enforce should be used. [Fowler]