Originally a term to make fun of students from the vicinity of Halle, Germany in the 18th century. This part of the country was famous for cultivating caraway (Kümmel) and was thus termed Kümmeltürkei. In modern times, only used as a derogative term for foreigners of middle eastern and Turkish descent, from the notion that Turkish people supposedly smell like Kümmel,caraway (or Kreuzkümmel, cumin, which is more typical of Turkish cooking); also onomatopoetically because of the high frequency of the umlaut "ü" in the Turkish language, e.g. in güle güle - "goodbye".
Compare Kümmelblättchen, in which the Kümmel- is derived from Hebrew gimel, "three".