Muscle Shirt Boy’s Costume. Choose this fun polyester costume and be ready to trick-or-treat, enter a costume contest or be the hit of the Halloween party! Wear this costume to a masquerade ball or as the perfect disguise at a haunted house. This outfit is sure to win you the "best costume" award at any Halloween costume party! Polyester with poly-fiber filling. Includes: Fabric shirt with distinctive muscle design and touch fastener closure Imported.