English
schmuck is from Yiddish שמאָק (
shmok) 'prick, dick; jerk, unpleasant person'. It has an unsure etymology. Many have noticed that German
Schmuck 'jewel' looks similar to the English spelling, and has an interesting meaning (cf. English
family jewels), but it is problematic. A
u in German corresponds to a
u (or an
i) and not an
o in Yiddish: cf. German
Fuß 'foot' with Yiddish
fus ~
fis, or German
gut 'good' with Yiddish
gut ~
git. (The English word is pronounced with a schwa, but the Yiddish one is pronounced definitely with an
o.) Pther etymologies connect Yiddish
shmok with an Old Polish word
smok 'grass snake, dragon' or a Czech word meaning 'fool'. he latter doesn't explain how a generic word for 'fool' came to be a specialized word for a body part. The other taboo words for the penis in Yiddish are (
daytshmerish, or high register)
shvants (cf German
Schwantz 'tail; penis' and Latin
penis 'tail; penis') and vulgar
pots. (Yiddish
pots can be used for the body part or a term of abuse. It is usually associated with German
Putz 'clean' (cf.
Putzfrau 'charlady, cleaning lady'), but has the same phonological problem as
shmok. Because
shmok and
pots tend to be associated with their term of abuse meanings, the body parts are usually refered to with endearing diminutives:
shmekl and
petsl. (The
u ~
i varience is for the two major dialect groups of Eastern Yiddish (spoken in the Pale), the Lithuanian (roughly NE Yiddish) and the Polish (roughly SE and divided into other dialect groups). It is well-known by non-linguists, and I have heard a song that is made up entirely using words that show the varience sung by a
polaks and a
litvaks. Here's
an article that discusses this in passing.)
[Edited for correction and minor style issue.]