Having the style or feeling of a film noir, which is a type of movie that is dark, mysterious, and often about crime.
The word 'noirish' makes people think of dark streets, secrets, and danger.
Examples:
The movie had a noirish feeling to it.
The book had a noirish cover.
The room was very noirish, with dark curtains and shadows.
The cat had a noirish look in its eyes.
The noirish city was full of secrets and danger.
The noirish music made me feel like something mysterious was going to happen.