Natural breasts that are so huge and horizontal in orientation that they could in theory be used as a shelf of sorts. Shelf Tities most often occur on obese black women, but on occasion can be seen on massive white and hispanic women. Shelf Tities must be natural, ie, huge breasts created via breast implants do not apply.
Guy 1: Hey man, look at that fat woman over there.
Guy 2: Holy Crap! those are some massive shelf tities!
Guy 1: Yea man, I bet you could set your drink up there and it wouldnt fall off.