"a pretty person or thing," 1736 of things, by 1773 of persons, from pretty (adj.). Prettinesses "pretty things, etc." is attested from 1640s.
"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" (from PIE root *kwo-, stem of relative and interrogative pronouns).
The sense developed from scholastic disputes over the nature of things. Original classical meaning "real essence or nature of a thing, that which distinguishes a thing from other things and makes it what it is" is attested in English from late 14c. (quidite).