Sleevage

The side of a boob or the space between a woman's breast and her rib cage that is revealed through the prevalence of overly generous sleeves in her clothing or, indeed, from a lack of sleeves entirely. It is a portmanteau, made-up from a combination of the words cleavage and side-boob.

My sleevage garnered some much-wanted attention on the train yesterday thanks to my cute new tank top.