This word — meaning roughly deep-bosomed — may be described as a companion to callipygian, which I investigated some time ago, though it is an even more rare and learned term.
One of the few writers to have used it was Oliver Wendell Holmes, who wrote in his Autocrat of the Breakfast-Table in 1858 that “The bathycolpian Heré ... sent down Iris”.
It derives from Greek bathus, deep, and kolpos, often translated as cleft or gulf (actually, by an indirect route through Italian, and following some mangling, it is the source of English gulf). It is sometimes written bathukolpian in an attempt to more closely represent the Greek spelling.
The Greek root kolpos had various senses, not only referring to the cleft between the breasts (and so the breasts themselves, as here), but also the cleft of the vagina and so by association the womb (hence the English medical prefix colpo–, as in words like colposcope for a surgical instrument used to examine the vagina and the cervix of the womb).