Malignant: 1. Tending to be severe and become progressively worse, as in malignant hypertension. 2. In regard to a tumor, having the properties of a malignancy that can invade and destroy nearby tissue and that may spread (metastasize) to other parts of the body.
The word malignant comes the Latin combination of "mal" meaning "bad" and "nascor" meaning "to be born"; malignant literally means "born to be bad."