English schmuck "A clumsy or stupid person" was borrowed from Yiddish שמאָק shmok "prick, dick; jerk, unpleasant person". I assumed this was cognate with German Schmuck "jewelry". It seems obvious, right? Family jewels and all that. But as zmejzhd points out, this connection is problematic because German u corresponds to Yiddish u, not Yiddish o. The American Heritage Dictionary states that the Yiddish word is probably borrowed from Old Polish smok "grass snake, dragon" - so its similarity to German Schmuck is a coincidence.
On the other hand, Calvert Watkins and the American Heritage Dictionary of Proto-Indo-European Roots state that English schmuck is "possibly" from Middle High German smuck, "clothing, adornment, jewels" (presumably thru Yiddish) from "Germanic *(s)muk-, referring to wetness and also to figurative slipperiness." Other words from Proto-Germanic *(s)muk- include Swedish smycken "jewelry" and English smock. (What is the connection between slipperiness and clothing/jewelry?)
The Proto-Indo-European root is *meug- "slimy, to slide". In addition to smock, it became Old English smēah, smēag "creeping, clever, sharp", smēagan "to think, to seek". Pokorny also lists sméagol "narrow, tight" (altho this is not in Bosworth and Toller). This must be where Tolkien got the name Sméagol.
The photo is from my trip to Berlin.