The lac- root is derived from Latin lac, probably related to Greek gala (the oblique forms of the latter are galakto- or galakso-).

A proto-form *g(a)lak- has been hypothesized; however, most other IE words related to "milk" seem to stem from a root *melg-. This is often interpreted to mean the "milk" word in both Latin and Greek was replaced by *g(a)lak- (there are other cases where both Latin and Greek have reflexes of a non-IE loan word); this explnation, however, is not universally accepted.