The phrase you're quoting is "Yiddishe kop", with the second word sounding more like "kawp". The word is actually straight from the German "kopf" meaning "head". A 'Yiddishe kop' is a "Jewish head". It's an observation expressed in the presence of a person who has just come out with an angle on the topic under discussion that is so audaciously unexpectedly logical in such an unconventional way that only a Jew, with 4000 years of the Jewish experience in his genes, could have thought to look at the subject that way ... whether brilliant or hopeless.