Etymology of monk dimin'?

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monk-dimin.jpgFirst of all, let me just say that I completely love Achewood. The comic is funny, clever, well written, hip, profane, and sometimes astonishing.
In a recent strip, Beef commiserates with Ray because Ray is going bald (or is it mange?), and he uses an expression to describe going bald: monk dimin’.
I did a little searching around and found several other references to the term, all meaning that same thing, but nothing in the urban dictionary and I’m coming up short trying to figure out where it comes from.
I assume it’s a reference to the monk’s tonsure, ok, but what’s diming? I’m tempted to think it’s actually from “diminishing” or something like that but it doesn’t add up.
Anyone care to hep me?