Malignant
Etymology
From Latin malignus, "wicked, of a bad nature," from male, "badly," paired with the same "born" root behind benign — so, roughly, "ill-natured." Medicine narrowed it to tumors that behave aggressively, spreading to other parts of the body. In ordinary use it still carries its older sense of active malice, as in "a malignant rumor."
adjective · /muh-LIG-nunt/
Dangerous and tending to grow, invade surrounding tissue, and spread — the medical sense of cancerous.
What it names
Why a diagnosis hinges on one word: whether a growth will stay contained or actively work its way through the body.
Example
Because the cells were malignant, doctors moved quickly to keep them from spreading.
Related idea
Benign is the safe opposite; metastasis names the spreading that makes malignancy so dangerous.
adjective · /muh-LIG-nunt/
Full of ill will — deeply malevolent and bent on harm.
What it names
spiteful intent or a poisonous influence
Example
A malignant rumor spread through the office.
Related idea
The opposite of benign; from Latin “malignus” (wicked).