Deadly  —adj. (-ier, -iest) 1 causing or able to cause fatal injury or serious damage. 2 intense, extreme (deadly dullness). 3 (of aim etc.) True; effective. 4 deathlike (deadly pale). 5 colloq. Dreary, dull. —adv. 1 like death; as if dead (deadly faint). 2 extremely (deadly serious).