In the second episode of the Peep Show, after having a discussion one character says to the other;
Shit is as shit does, my friend.
YouTube link:
The End Of "The Hair Blair Bunch" at 0:42
I couldn't grasp the grammatical structure and the meaning of the sentence. I searched it on the Internet but maybe because it's a specific sentence, I couldn't find anything relevant.
I don't know the expression but I would interpret it much along the lines of you are what you eat. It's what you do (or how you act) that defines you.
On the other hand, the expression x is as x does is common. If you google is as does you will come across a raft of explanations of what similar comparisons might mean.
This is a version of a very old proverb "handsome is as handsome does".
The grammar and vocabulary of the proverb are both archaic.
A literal modern version would be something like
A good person is somebody who behaves considerately.
("Handsome" could have several meanings, so people might disagree with my choices of how to translate it, but the structure of the sentence is clear).
