the prefix dis- is not always a negating prefix

This illustrates how many normative grammarians are misled by extra-linguistic considerations into manhandling the language. The notion that language must be consistent in the distribution of its vocabulary. This leads to concerns such as that in this thread. That the form disgruntled lacks a positive without the prefix. One can also note that the past participle lacks a verb: gruntelen 'to grunt (frequently)' which dropped out of the language during the Middle English period.

Ceci n'est pas un seing.