early 15c. skimmen, "lift the scum from by a sliding motion, clear (a liquid) from matter floating on the surface" (the agent noun skimmer, for the utensil used, is attested from late 14c.), from Old French escumer "remove scum," from escume (Modern French écume) "scum," from a Germanic source (compare Old High German scum "scum," German Schaum; see scum).
The meaning "throw (a stone) so as to skip across the surface of (water)" is from 1610s. The meaning "move lightly and rapidly over the surface of" is from 1650s, from the motion involved in skimming liquid; that of "glance over carelessly, pass over lightly in perusal" (in reference to printed matter) is recorded by 1799. Related: Skimmed; skimming.