quiddity (n.) Look up quiddity at Dictionary.com
1530s, "captious nicety in argument" from Medieval Latin quidditas, literally "whatness," from Latin quid "what," neuter of indefinite pronoun quis "somebody, someone or other" (see who). Sense developed from scholastic disputes over the nature of things. Original meaning "real essence or nature of a thing" is attested in English from 1560s.