Sinister (adjective)

1. Suggesting something evil, wicked, or harmful.
2. Threatening or dangerous.

Origin:

From latin sinister "on the left, unlucky", from sinistra "left hand, left side". .

Examples:

1. He had a sinister look in his eyes.
2. The old house had a sinister atmosphere.
3. The government's policies have a sinister motive.
4. The company's new product has a sinister side effect.
5. The man had a sinister reputation in the community.