how old skool writers,hip hop dudes used to say dope,sick,sweet,ill, etc...etc.. Some one will probably look at you with big eyes if you say it today.. Unless your 62 years old.
The word def is hip hop (including graffiti) slang for 'great,' 'ill,' 'dope,' 'fly'--generally, a positive adjective. Taken from the root word 'definitive,' if something is 'def' then it is classified as being very good. If something is very definitive in its representation of its genre or classification, it is 'def.'