ded'-li: In the Old Testament two words are used in the sense of a "mortal (Hebrew nephesh, "hateful," "foul") enemy" (Ps 17:9), and in the sense of "fatal disease," the destructiveness of which causes a general panic (Hebrew maweth, "death," 1Sa 5:11).
⇒See the definition of deadly in the KJV Dictionary
In the New Testament we have in Re 13:3,12 the expression "deadly wound" (Greek thanatos), better "death-stroke," as in the Revised Version (British and American), and the phrases "deadly thing," i.e. poison (thanasimon ti, Mr 16:18), and "full of deadly poison" (meste iou thanatephorou, Jas 3:8), said of an unruly tongue. Both Greek words convey the idea of "causing or bringing death" and occur in classical literature in a variety of uses in combination with the bite of venomous reptiles, deadly potions, mortal wounds and fatal contagion.
H. L. E. Luering