Schmaltz — rendered poultry fat — essential to northern European Jews forbidden to use lard or butter for cooking, and without access to olive or sesame oils, comes from the word for “tallow.” People with the surname Schmaltz are not named for chicken fat but for the profession of being candle makers, early candles being made from tallow.
Creating good schmaltz requires having a fat bird, and creating fat birds has been something people have been doing for at least 4,500 years by force-feeding them grain. Migrating birds (such as geese) normally store fat in their livers before taking to the air for that long trip. People quickly discovered a delicious by-product of this process: the fattened liver. Fois gras became a specialty of Jewish cooks who were known to produce the biggest livers. Whether or not eating part of an animal produced by force-feeding wasn’t considered much of an issue until relatively recently when people who are perfectly happy eating animals raised under equally horrible conditions felt bad for the geese. Such people can be said to be "schmaltzy," or overly sentimental.
After the fall of the Roman Empire, it was these Jewish cooks who maintained the tradition of producing foie gras in Europe. The liver, being primarily an organ of blood, does, however pose difficulties for the kosher cook, and though it is possible to treat the liver so that it meets Kashrut standards, it is prohibitively expensive and rather involved. Far simpler it is to use chicken livers instead.
The Kosher Cookbook Trilogy, Ruth and Bob Grossman, 1963