layer (n.)
late 14c., "one who or that lays" (especially stones, "a mason"), agent noun from lay (v.). Passive sense of "a thickness of some material laid over a surface" is first recorded 1610s, but because the earliest English use was in cookery this is perhaps from French liue "binding," used of a thickened sauce. Of hens from 1707. Layer cake attested from 1875.
layer (v.)
1832, in gardening, as a method of plant propagation, from layer (n.). Meaning "to form into layers" is from 1852. Related: Layered; layering.