Facetiae is a plural noun that refers to witty sayings, clever jokes, and humorous writings. If you have a collection of joke books on your shelf, you have facetiae.
Facetiae comes from the Latin word facetia, meaning "wit" or "cleverness." The word describes a genre of literature focused on humor and wit. Collections of facetiae were quite popular during the Renaissance. Educated readers enjoyed the short, pithy anecdotes and sharp-witted observations found in such collections. In later times, the term became a euphemism used in libraries as a label for more edgy writings or ribald humor. Today, a collection of facetiae likely includes works with a sophisticated style of comedy relying more on wordplay and irony than simple slapstick.