costly /ˈkɒst.li/adjective 1noun 1forms 4 adjective1 •Of high cost; expensive.Individual mistakes proved costly for Wigan who, particularly after the half-time introduction of Hugo Rodallega, dominated for long periods. — 2011 noun1 •Short for costly colours.Costly is a simpler game than cribbage. formscostlier · more costly · costliest · most costly