1660s, "process of suckling an infant," from Fr. lactation, from L. lactationem (nom. lactatio) "a suckling," from L. lactatus, pp. of lactare "suckle," from lac (gen. lactis) "milk," from PIE base *glact- (cf. Gk. gala, gen. galaktos "milk"). Meaning "process of secreting milk from the breasts" first recorded 1857. M.Ir. lacht, Welsh llaeth "milk" are loan words from Latin.