First written down in the early 1600s, in its obsolete form kenspeck, though it was likely in oral use for a while before that. It probably came from Scandinavian roots -- compare Old Norse kennispecki, power of recognition, and modern Norwegian kjennespak, quick at recognizing, lit. know-clever. You ken? (meaning, know/understand/perceive)
You may think yourself "geyan fine," all covered with Scotch plaid like that, but I wouldn't be so kenspeckle for worlds!