said to be from Pamphila, a Greek lady, whose chief work is a commonplace book of anecdotes, epitomes, notes, etc. Dr. Johnson suggests par-un-filet (held “by a thread”) —i.e. stitched, but not bound; another derivation is paginae filatae (pages tacked together). It was anciently written panfletus, pamflete, and by Caxton paunflet.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894