costumer UK /ˈkɒstjuːmə(ɹ)/US /ˈkɑstumɚ/ noun4 •A person who designs, makes or supplies theatrical costumes. •A person who wears a costume. •One who takes part in cosplay. •(informal) A costume drama.The success of this lush costumer encouraged Woo to try his hand at comedy. — 2000 formscostumers