Pronunciation: (līf'lis),[key] —adj. 1. not endowed with life; having no life; inanimate: lifeless matter. 2. destitute of living things: a lifeless planet. 3. deprived of life; dead: a battlefield strewn with lifeless bodies. 4. without animation, liveliness, or spirit; dull; colorless; torpid: a lifeless performance of a play. 5. insensible, as a person who has fainted.