Learn English – the origin of “can’t into”

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.

It's a sarcastic response, a short and colloquial version of "Do you think?"

In its straightforward form, it's just a request for affirmation. And it's probably impossible to gauge when the ironical version crept in and took over. Nevertheless, an NGram search shows it beginning in the 1970s and taking off in the '90s, so my guess is that's when it gained wide acceptance as a recognized trope in the sense you mean.