I looked up “idiosyncratic” in The American Heritage® Dictionary of the English Language: Fourth Edition. 2000 and received the following definition:

idiosyncrasy

SYLLABICATION: id·i·o·syn·cra·sy
1. A structural or behavioral characteristic peculiar to an individual or group.
2. A physiological or temperamental peculiarity.
3. An unusual individual reaction to food or a drug.

ETYMOLOGY: Greek idiosunkrsi : idio-, idio- + sunkrsis, mixture, temperament

I never knew that an idiosyncrasy was also a type of allergy. Looking up “idiosyncrasy” in the OED, it appears that the word originally referred to physical temperment, and not mental peculiarity.