Old English deadlic "mortal, subject to death," also "causing death;" see dead + -ly (1). Meaning "having the capacity to kill" is from late 14c. (Old English words for this included deaðbærlic, deaðberende).
1. His mother's voice was one he knew; ice cold and deadly.
2. Broadcast news was accurate and reliable but deadly dull.
3. She took the gun and blazed away with calm and deadly accuracy.
4. The authorities are looking into last week's deadly gas explosions.