I was going to suggest Dictionaries.io but it doesn't fit your request of having slang available.
Also, I read somewhere that some files for a wordprocessor could be made into being available in Apple's own Dictionary.app - "Stardict" something - I forget. But, I'll see if I can look through my bookmarks and find anything more helpful about this. I can't say if these will add slang, but I just wanted to share the knowledge I have.
Ok so pls forget slang, dictionaries.io is better than apple app?