The term 'RIBS UNIT' is composed of two words, 'RACK' and 'SLAB'. The word 'RACK' is a four-letter noun that refers to a framework or stand on which objects are placed. It can also refer to a set of parallel bars or beams used to support a roof. The word 'SLAB' is also a four-letter noun that refers to a large, thick, flat piece of stone, concrete, or other material. Together, the two words 'RIBS UNIT' refer to a unit composed of a framework or stand with a large, thick, flat piece of material attached to it. This type of unit is often used in construction, such as for roofing or flooring.