There's at least three etymologies. The one you usually see is to connect it with German Schmuck 'ornament, decoration, jewels' (English smock is cognate with the German word); another one is in Kluge, from Slovenian smok 'fool'; and finally an origin in Slavic smok 'snake, dragon' (this is the one that the A-H gives). Of the two Yiddish dictionaries I have, neither Weinreich and Harvaky list the word. (This is weird since Harkavy isn't that squeamish and does list shmue 'cunt' and pots (pl. pets) 'penis; fool.) The vowel is problematic. I've always heard the word pronounced as /Sm@k/ and never /Smuk/, but then pots came into English as /p@ts/, too. shmuts 'dirt, filth' came in as /Smuts/, so I think that the Yiddish word should be shmok. I'm just not sure, but I lean toward the 'snake, dragon' rather than the 'jewels' etymology.