When something is painkilling, it can make pain smaller, or take the pain away. People use this word almost only about medicine: a painkilling drug, a painkilling injection, a painkilling cream. It sits next to the noun painkiller, which is the medicine itself; painkilling tells you what that medicine does. You will hear it in hospitals, on boxes, and in news about health more than in ordinary chat. In everyday talk people often just say it is “for pain,” or they name the medicine. It is a fairly medical, slightly formal word. People do not usually call a hug or a kind word painkilling, even though those can ease hurt too — the word stays with drugs and treatment.