1) Used to describe a female’s very attractive body (adj.)
2) The act of having sex. Pronounced with a pause: "Rock---It" (verb)
The first usage is derived from the second. The term "Rock-It" has long been used to describe the act of man having sex with a woman. Where the man in "rockin" the woman. The second term is used to say that a woman has a body that is worth "rocking".
1) Dan, look at Lizzy, she has a rocket body.
Meaning: Dan, look at Lizzy, she is a very attractive woman.
2) Lizzy is hot. I am telling you Dan, I just want to rocket.
Meaning: Dan, I want to have sex with Lizzy because she is hot.