also repellant, "having the effect of repelling," 1640s, from Latin repellentem (nominative repelens), present participle of repellere (see repel). Originally of medicines that reduce tumors; the meaning "morally repelling, repulsive, distasteful, disagreeable" is recorded by 1797.
That is repellent which keeps one at arm's length ; that is repulsive from which one recoils ; that is, the second is a much stronger word. [Fowler, 1926]
By 1805 in the specific sense of "capable of repelling water, impervious to moisture."
also repellant, 1660s, "agent or medicine that reduces tumors," from repellent (adj.). As "substance that repels insects," 1908.