The Collaborative International Dictionary of English v.0.48:

Flip-flap \Flip"-flap`\, n. [See Flip, and Flap.]
   The repeated stroke of something long and loose. --Johnson.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flip-flap \Flip"-flap`\, adv.
   With repeated strokes and noise, as of something long and
   loose. --Ash.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flip-flap
    adv 1: with repeated strokes and noise; "something going flip-
           flap in the night"