Capitalized, Flibbertigibbet, according to Webster 1979, is the name of the devil. The same source notes that it is "obsolete." In lower case, it can refer to:
  1. an imp; one who looks impish
  2. a person who chatters constantly
  3. an irresponsible, flighty person
"Flibbertigibbet is from Middle English flipergebet, which is probably an imitation of the sound of meaningless chatter. The word originally meant a gossip or chatterbox, but it soon took on the idea of a light-minded or frivolous person, especially a pert young woman with such qualities. Flibbertigibbet was also the name of an imp or demon; it appears in a list of 40 fiends in a book by Samuel Harsnet, and also in Shakespeare."*

let's add what Webster 1913 left out