Quiz: Remember the definition of mal de mer?
1570s, "having a meaning," from Latin significantem (nominative significans, present participle of significare "make known, indicate" (see signify). Earlier in the same sense was significative (c.1400). Often "having a special or secret meaning," hence "important" (1761). Related: Significantly. Significant figure is from 1680s. Significant other (n.) attested by 1961, in psychology, "the most influential other person in the patient's world."