The expression "Pitch a loaf"

I think the phrase you're looking for is actually pinch a loaf. This forum discussed its origins, with one person writing:

Amazingly enough, none of my references give the history of "pinch a loaf." But I'm guessing that the process of kneading and shaping dough gave rise to the expression.

This slang dictionary supports your intuition about what it means:

verb

to defecate.

He's in the bathroom pinching a loaf.

Another slang dictionary writes that the phrase is from 1994.

I have never, however, heard this phrase used. I'm an American English speaker, and at least in my experience this euphemism is not popular.

As none of the other answers posted has identified the probable country of origin of this expression, I note that Jonathon Green, Chambers Slang Dictionary (2008) has this brief entry for the phrase:

pinch a loaf v. {1990s+} (US) to defecate

And Tom Dalzell & Terry Victor, The New Partridge Dictionary of Slang and Unconventional English (2006) has this:

pinch a loaf to defecate US [Cited source:] Michael Dalton Johnson, Talking Trash with Redd Foxx, p. 44, 1994

This tends to confirm that there is no connection between the meaning in question and any sense along the lines of "steal bread," since "pinch" as slang for "steal" is primarily a British English slang usage. Without (I hope) plunging too graphically into the metaphor at work here, I think that the phrase uses "pinch" in the sense of "pinch off," with the sphincter as the pincher. I have never heard this expression used.

The expression means "to take a big crap". For example:

Man, after that McDonalds I need to pitch a loaf.

For the origin, The Phrase Finder forum says:

Amazingly enough, none of my references give the history of "pinch a loaf."