Here's what it might mean, depending on the context:
* Something covered in glitter: This is the most likely meaning, especially if you're talking about clothing, decorations, or other objects.
* A person who loves glitter: This could be someone who wears a lot of glitter, decorates their home with it, or just enjoys the look and feel of it.
* A term related to a specific subculture or interest: Some online communities might use "glitterish" to describe a particular aesthetic or style related to glitter.
Without more context, it's impossible to say for sure what "glitterish" means. If you provide more information about where you encountered the word, I might be able to give you a more specific answer.