lacto- Look up lacto- at
before vowels, lac-, word-forming element used in chemistry and physiology from 19c. and meaning "milk," from Latin lac (genitive lactis) "milk," from PIE root *glakt-, source also of Greek gala, genitive galaktos, "milk." This and the separate root *melg- (see milk (n.)) account for words for "milk" in most of the Indo-European languages. The absence of a common word for it is considered a mystery. Middle Irish lacht, Welsh llaeth "milk" are loan words from Latin.