defoliant (n.)

"deprive of leaves, strip the leaves from," 1793, perhaps a back-formation from defoliation. Earlier in this sense was defoil (c. 1600), which was identical in form with a verb meaning "to trample underfoot." Related: Defoliated; defoliating.

agent or instrumental suffix, from Old French and French -ant, from Latin -antem, accusative of -ans, present-participle suffix of many Latin verbs. Compare -ance.