slab-sided \ˈ ̷ ̷| ̷ ̷ ̷ ̷\adjective : having the rib cage narrow from side to side with flat sides and the ribs deficient in lateral curvature < a slab-sided horse > also: giving an effect of such a condition by reason of leanness or slenderness < an airplane with a slab-sided fuselage > < slab-sided cheeks >