noun pl. idiosyncrasies -·sies

1. the temperament or mental constitution peculiar to a person or group
2. any personal peculiarity, mannerism, etc.
3. an individual reaction to a drug, food, etc. that is different from the reaction of most people

Etymology: Gr idiosynkrasia < idio-, one's own, peculiar (see idio-) + synkrasis, a mixing together, tempering < synkerannynai, to mix together < syn-, together + kerannynai, to mix < IE *ere-, to mix > rare, Ger rühren, to stir