possessing the appearance of doom; when something looks frightening, dark, dreary, gray, dangerous, or just plain scary.
When the sky gets really dark and cloudy, during a tornado, tsunami, hurricane, or thunderstorm, you can say it looks doominous.
When talking about a scary Freddie Crougar looking person with a meat cleaver.. "He looks doominous."
Like the Hash Slinging Slasher..
or a huge math final (or teacher).