The word "flibbertigibbet" was a word originally used to describe whimsical and flighty people, and it was mainly used to describe women at the time. In modern times, it is often used as a slang word to describe any person that gossips or talks too much. 
The word is supposedly of Middle English origin, and the pronunciation is most likely based off the sounds of meaningless chatter.  The word was originally spelled "flepergebet."
Its earliest use was also recorded sometime in the late 15th century. The word was most notably used in Shakespeare's "King Lear" in 1605 to describe an evil spirit or fiend. It is said that he was inspired by a character from "Declaration of Egregious Popish Impostures", which was written by Samuel Harsnett in 1603.
Some common synonyms for flibbertigibbet include ditz, cuckoo, scatterbrain, and nitwit.