From an old column of mine:

According to the “Philologos” (word lover) column in the Jewish newspaper The Forward, the word “Jew” is unique in the English language. In English, there are two rules of thumb for using nouns as adjectives when referring to ethnicity or geographic origin.
In one system, the noun and the adjective are the same. “Russian,” “German” and “Greek” are interchangeable as nouns and adjectives; you say, “he’s Greek” and “he’s eating a Greek salad.” In the second system, the noun and the adjective are different. You drink “Turkish” coffee, but no one says “he is a Turkish.”
“There is, to the best of my knowledge,” according to Philologos, “only one consistent exception to this rule: the word Jew.” You can use “Jew” and “Jewish” interchangeably. The column contends that using “Jew” as an adjective is therefore offensive because it inherently suggests that Jews are “unlike all other human beings.”
If you call someone a “Jewish lawyer” you’re being descriptive and grammatically correct. Call someone a “Jew lawyer” and you’re being grammatically correct and you’re suggesting that a Jewish lawyer can never be anything but a Jew. A “Jewish boy” is some kid named Sol you pat on the head. A “Jew boy” is something Nazis say.
It’s kind of funny that “Jew” is being rehabilitated largely – I suspect – because “Jewish person” is too long for newspaper headlines and the TV news. Nevertheless, even though I hope Lieberman meets electoral defeat, I’m grateful that he’s helped elevate a word that should never have been lowered in the first place.