Hip-Hop Lingo. Phrase originated circa 1985. In use from 1985-1987. Most popular usage was in M.C. Shy D's 1986 hit Gotta Be Tough. Generally an expression used within description when someone hears a great song (or encounters greatness through any sort of entertainment medium) and they need to tell somebody about it, usually somebody with clout or power.

I'm considering making it my personal crusade to put this phrase back into popular usage. It's just too fun to say.

It's pretty abvious but it took me some time to figure it out.
What does number 3 on the phone have to do with the word def?

Take a look at number 3 on the phone:

-------------------
|  1  |  2  |  3  |
|     | ABC | DEF |
------+-----+------
|  1  |  2  |  3  |
| GHI | JLK | MNO |
------+-----+------
|  1  |  2  |  3  |
|PQRS | TUV |WXYZ |
-------------------

I like this phrase.
It's plain stupid and just not to geeky so anyone can understand it.