I believe the saying comes from Frederic Goudy who once said "Anyone who would letterspace blackletter would steal sheep." Although it is commonly misquoted as "Anyone who would letterspace lower case would steal sheep."

I can't remember why, I think someone gave him an award with letterspaced blackletter that he did not like.

OK! So I guess one could translate it into: Letterspacing lowercase(/blackletter) is bad typography manners?

Thank you!

From what I understand Goudy said that anyone letterspacing blackletter would be the kind of person who'd pull a Gill on the poor sheep.
As we don't use blackletter much these days and the reference to zoophilia would steal the focus from the typographic lesson to be learned, a cleaned up version is now the common one.

The Stop Stealing Sheep book tells the story, if I remember correctly. The original verb Goudy used was shag. And also of note, letterspacing blackletter was a established way of adding emphasis. But yeah, bad manners is the implied meaning.

Thanks again!
So, fredo... You think there was also a clear reference to Gill's zoophilia in that sentence or...?

No - Goudy had died many decades before Gill's zoophilic experiments became publicly known.

Yes, the Gill reference was my very own.

Thanks guys!