Deh can mean good, bad, or mid. Like how aloha means hello and goodbye. When using this you have to use it in the right context, or you will leave everyone being very confused. No one really knows where it came from, all we know is that it originatedfrom the depths of Snapchat.
"deh" is a word (adj, noun, verb, pronoun, addverb, ect.) that is used by complete assholes when they cannot think of anything else to say in a conversation, simply because they are stupid and/or don't have the balls to tell you what they really feel!