adjective, dead·li·er, dead·li·est.
- causing or tending to cause death; fatal; lethal: a deadly poison.
- aiming to kill or destroy; implacable: a deadly enemy.
- like death: a deadly pallor.
- excruciatingly boring: The dinner party was absolutely deadly.
- excessive; inordinate: deadly haste.
- extremely accurate: Annie Oakley was a deadly shot.
- in a manner resembling or suggesting death: deadly pale.
- excessively; completely: deadly dull.
adjective -lier or -liest
- likely to cause deathdeadly poison; deadly combat
- informal extremely boring
- like death in appearance or certaintydeadly pale; a deadly sleep
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).