Any smokin' hot chick or chicks. Originated from "robble grobble," the incoherent shouting sounds made during crowd or mob scenes in the television show South Park. An effective phrase to use when trying to look busy working while checking out chicks from your office window. Shortened to "grobble" to more easily indicate that a hot girl is approaching and interested men should take heed.
Look to your right, there is some smokin' grobble over there. Hot damn!