The specific word "cleavage" that you speak of is a derivation of the word "cleave".
Cleave "split" comes ultimately from Old English cliofan/cleofan. There were cognates in Old Scandinavian, Old High German, and Old Norse. The Old Teutonic root was *kleub-, and we even get a pre-Teutonic root with this one: *gleubh! Both meant "split".
The pre-Teutonic root is thought to be the source of Greek gluph- "cut with a knife" and Latin glub- "peel, flay". The Old English inflected forms became cleave, clove and cloven, though cleft appeared in the 14th century and has survived.
Cleavage originally was a technical term in geology (1816). The sense of "cleft between a woman's breasts in low-cut clothing" is first recorded in 1946, when it was defined in a "Time" magazine article as the "Johnston Office trade term for the shadowed depression dividing an actress' bosom into two distinct sections."
http://vernondent.blogspot.com/2007/07/because-of-google-traffic-thats-why.html