early 15c., "effective as a cause or agent," from Old French causatif, from Latin causativus, from causa "a cause, reason" (see cause (n.)). Meaning "expressing causation" is from c. 1600; hence the noun, in grammar, "a form of a noun or verb expressing causation" (1824).
1820, "one who or that which rustles" (a leaf, a bird), agent noun from rustle (v.). The American English meaning "cattle thief" is by 1882; earlier it meant "active, efficient person" (1872).
also co-efficient, c. 1600, "that which unites in action with something else to produce a given effect," from co- + efficient. Probably influenced by Modern Latin coefficiens, which was used in mathematics in 16c., introduced by French mathematician François Viète (1540-1603). As an adjective, "acting in union to the same end," from 1660s. Related: Coefficiency.