Green’s Dictionary of Slang

1. the penis [it ‘flaps’ around].

1653		Urquhart (trans.) Gargantua and Pantagruel (1927) I Bk I 51: I would have cleft her water-gap, / And join’d it close to my flip-flap.
1890–1904		Farmer & Henley Sl. and Its Analogues.
c.1904		A. Crowley Snowdrops from a Curate’s Garden 25: The amorous and quick-witted lad, placing his feet together, made a capital catch-‘em-alive-o for his horny flip-flap.

2. copulation.

1702		T. Brown Letters from the Dead to the Living in Works (1760) II 265: Those poor girls, who have nothing to depend on but the drudgery of flip-flap. [...] A horn-mad cuckold, that had caught his wife playing at flip-flap with her tail like a live flounder in a frying-pan.
1770		Foote Lame Lover in Works (1799) II 66: The Marquis of Cully and Fanny Flip-flap the French dancer.

3. a broad fringe of hair falling across the forehead, esp. as used by street boys [it ‘flaps’ around].

1909

J. Ware Passing Eng. of the Victorian Era 134/2: Flip-flap (Street boy, 1898 on). Broad fringe of hair covering the young male forehead. This fashion, revived from the time of George IV., began with the quiff [...] expanded to the guiver, and widened to the flip-flap, a name evidently gained from its motion in the winds.