quid (n.2) “one pound sterling,” 1680s, British slang, possibly from quid “that which is, essence,” (c.1600, see quiddity), as used in quid pro quo (q.v.), or directly from Latin quid “what, something, anything.” Compare French quibus, noted in Barrêre’s dictionary of French argot (1889) for “money, cash,” said to be short for quibus fiunt omnia.
“Quiddity?” I wondered.
quiddity (n.) “a trifling nicety in argument, a quibble,” 1530s, from Medieval Latin quidditas
“the essence of things,” in Scholastic philosophy, “that which
distinguishes a thing from other things,” 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 classical meaning “real essence or nature of a thing” is
attested in English from late 14c.