Online etymology says "informer," 1785, probably from underworld slang meaning "the nose" (1700), which apparently developed from an earlier meaning "fillip on the nose" (1670s). Snitcher in same sense is from 1827. More etymologically-inclined persons might be able to share more light on it!

So it's not far off the date, but perhaps a bit too slangy for a Prince who grew up in Rome.

Definitely getting there, though.