Things that are seamy have lost all morality. Your mother wouldn't approve of you going to a seamy video arcade, especially not if you went with a group of seamy people.
The seamy people at that video arcade could also be called "sleazy" or “a bad crowd.” A house that’s falling down could also be seamy, and so is the bar on the corner where not even your uncle will go. If you find yourself in a seamy situation, the best thing to do is get out before you find yourself turning seamy, corrupted by the seamy folk lurking about.
