1650s, "pertaining to milk," earlier "milk-white" (1630s), from Latin lacteus "milky" (from lac "milk," from PIE root *g(a)lag- "milk") + -al (1). Other 17c. attempts at an adjective in English yielded lactary, lactaceous, lacteant, lacteous, lactescent, and, in a specialized sense ("milk-producing"), lactific.
