It might have participated in some great Shakespearean insults – if only it had come into its own just a couple of centuries earlier.
In the late 18th century, schist (a layered type of metamorphic rock) was hewn from the earlier schistus, modified at the beginning of the 17th century from le French, schiste.
The French had broken up Latin’s schistos lapis (“stone that splits easily”~ Pliny), in turn derived from Greek skhistos (divided, separated) – originally a chip of their ol’ skhizein (to split).
No surprises then that our rock-star schist is described as splitting easily into layers. And that’s apt because I rather enjoy saying it aloud in any of its various forms, schistous, schistic, even schistaceous from about 1900 (having the colour of a blue-grey schist or slate).
Slate is a similar type of rock though a less metamorphosed variety. And this is worth knowing because of the characteristic layering; called schistosity in schist but slaty cleavage in slate.
That’s right, cleavage (dating back to 1816) was sported by the scientists for well over 100 years before the fashion world and its censors dressed its definition up (or down, depending on your perspective). Thus cleavage has only described the cleft between our boobies since the 1940’s.
Sources: Online Etymology Dictionary, Oxford English Dictionary