Source : Webster's Revised Unabridged Dictionary (1913)

Flip-flap \Flip"-flap`\, n. [See {Flip}, and {Flap}.]
   The repeated stroke of something long and loose. --Johnson.

Flip-flap \Flip"-flap`\, adv.
   With repeated strokes and noise, as of something long and
   loose. --Ash.

Source : WordNet®

flip-flap
     adv : with repeated strokes and noise; "something going flip-flap
           in the night"