Forbidding (adjective)

1. Having an intimidating or unfriendly appearance that discourages approach or approachability.
2. Appearing difficult or impossible to do or accomplish.

Origin:

From the middle english word "forbidde" meaning to prohibit or to command against something.

Examples:

1. The old castle had a forbidding appearance, with its high walls and dark windows.
2. The storm clouds had a forbidding look, as if it was about to rain.
3. The mountain had a forbidding peak that no one had ever climbed.
4. The teacher had a forbidding expression on her face, and the students knew they were in trouble.
5. The task seemed forbidding, but he was determined to finish it.