it's one of those words that Pete defines, and then gives examples of the trope, rather than citations for the word itself

It's one of the things that separates the cream from the chaff, lexicographically speaking.

For what it's worth, I took a quick spin through oneof the editions of Du Cange's Glossarium mediae et infimae Latinitatis, digitized by and online at the National French Library, but found nothing under soeresmus or soresmus.

Ceci n'est pas un seing.