floating rib 
noun
Any of the lower ribs which are not attached directly to the breastbone. Also called false rib.The tenth rib may be a floating rib (a frequent observation by Japanese investigators)....
- The focus is on lengthening the space between the hip bones and the lower, floating ribs - the area where the baby is growing and the mother's internal organs are rising and shifting - and on opening the pelvis.
- Here toe kicks should be directed at the larger, most accessible areas - such as the lower abdomen and floating ribs.