Cacatum non est pictum - a Question

This from an email from a guy on the quizbowl circuit:

“While working on a piece of Unheard Beethoven just scavenged from the British Library, we ran across a canon by Beethoven on the phrase “Cacatum non est pictum.” I’ve picked up enough off the web to recognize it as being slightly scatological, but can someone give me a proper translation and a source from which it comes? I know that it appears in a poem by Heine (Deutschland, Caput XI), but that’s too late to be Beethoven’s source (the canon is from 1815 and the poem 1844). Many thanks!!!”

Anyone wanna take a stab at this?

Who knows . . . maybe the guy who wrote this email is on this MB.

I have no idea where it comes from, but
cacatum = past participle of cacare, to shit
pictum = past participle of pingere, to paint

so the phrase could be translated as:
[that which has been] shat is not painted (?)

In other words, I have no idea what the phrase is supposed to mean. I found the poem by Heine but the context didn’t help me much. The poem says “this is what we would have told Cornelius”, where Cornelius (presumably) is the roman author of a biography of roman celebrities, “De viris illustribus.”

I am under the impression that it basically means “Nobody ever shat gold” or something equally valuable/pretty. IOW, war sucks, and the end doesn’t always justify the means. Again, IOW, don’t complain about how ugly war is if you win.

The context of the original quote would help but, in the meanwhile here’s my WAG:

Shit (bad stuff) is not painted (described).

Or, in other words, In what I said, I left out the bad stuff. This could be used in two contexts: one to convey that if you think what I just told you is bad, I left out the really bad stuff. Or, another: In my description I told you the good stuff and left out the bad.

my WAG: it’s a description of a painting, in the vein of “That was defecated, not painted.”

The poem tells the Germans were successful in rejecting the Romans and describes what history would have been had the Romans succeeded in conquering them.

Something like saying today “had the Germans won the war, we’d all be speaking German today”.