adj. & adv.
adj. (deadlier, deadliest) 1 a causing or able to cause fatal injury or serious damage. b poisonous (deadly snake). 2 intense, extreme (deadly dullness). 3 (of an aim etc.) extremely accurate or effective. 4 deathlike (deadly pale; deadly faintness; deadly gloom). 5 colloq. dreary, dull. 6 implacable.
adv. 1 like death; as if dead (deadly faint). 2 extremely, intensely (deadly serious). deadly nightshade = BELLADONNA. deadly sin a sin regarded as leading to damnation, esp. pride, covetousness, lust, gluttony, envy, anger, and sloth. deadliness n.
[ OE deadlic, deadlice (as DEAD, -LY(1)) ]