In Yehoash's Tanakh, we came across this word (without 'keyn' and without 'on a' before it). Neither your dictionary nor Weinreich's showed the meaning.
We did find it in Harkavy's dictionary: any
Even if that meaning is now obsolete, I think your dictionary should include that meaning and show the status as obsolete.
You would likely want to add text to Section B 5.4 of your user's guide. For example: "We have included some words that are now obsolete in Yiddish because some readers may read older literature in which these words appear; such words should not be used now."