Cancerous describes something related to cancer, a serious disease where abnormal cells in the body grow uncontrollably and can spread to other parts of the body. When doctors say a tumor is cancerous, they mean it contains these dangerous cells that need treatment. Unlike benign tumors, which stay in one place and don't spread, cancerous tumors can invade nearby tissues and organs.

People also use cancerous figuratively to describe something harmful that spreads and gets worse over time, like destructive gossip or corruption in an organization. When someone says a problem is “like a cancer,” they mean it's harmful in itself and also threatens to spread and damage everything around it. A cancerous rumor might start small but grow as more people repeat and exaggerate it, causing increasing harm.

The medical meaning is always serious: cancerous cells threaten health and life. The figurative meaning compares that same quality to something dangerous that spreads and worsens if left unchecked.