What the word "freak" does really mean

Today I have known learnt/discovered one interesting thing about this word. "Freak" nowadays is using in Russian too ("фрик"). I thought that this word means "monster" or something similar (very strange man, for example). But it isn't just that. Freak is someone who is obsessed with something. And if you don't understand his or her interest you could say "freak" - "you are a fitness freak" or "a baseball freak". I think it's an informal phrase word and you musn't use it in business communication, just with close friends (or not close at all, or with enemies too).
I would like to read a book named "Vegan Freak". I guess it is interesting. And now I have a new understanding of its name.  (languages are living and words are adapted and adopted to new uses; a freak is likely to be able to freak you out too. It also means a vagary/caprice/something beyond nature; a freak event, a freak of nature; Ussain Bolt is one freakish athlete)

Please correct me if I made mistakes! English grammar makes me cry sometimes ...but I pull myself together and continue learning :) Don't get freaked out whatever you do!