Mmk isn't really
from anything in particular. I've heard lots of people say it in person and in movies/tv.
The more I type mmk the weirder it looks. Here's an audio example-
[ame="http://www.youtube.com/watch?v=E4_tOiLB_Ko&feature=related"]YouTube - Mmkaay[/ame]
I love South Park sometimes
Also since I brought it up earlier-
[ame="http://www.youtube.com/watch?v=HXnUZBD_qV4"]YouTube- South Park - Mr. Mackey mmmkay ?[/ame]
again