Green’s Dictionary of Slang

1. (orig. US) a very young prostitute.

1889–90

Barrère & Leland Dict. of Sl., Jargon and Cant I 372/1: Flippers, flappers, very young girls trained to vice, generally for the amusement of elderly men.

2. (US) a male flapper n.² (2)

1905		Richmond River Exp. (NSW) 9 June 12/1: Flipper: Why does ho object to his wife going out alone In her motor? Flapper: Because he can’t see how one unmanageable thing can manage another.
1920		T. Thursday ‘Mr. Mister’ in All-Story Weekly 22 May 🌐 I’m positive that the flipper who thinks he’s It, tagged himself by slanting in a mirror.
1922		Appleton Post-Crescent (WI) 3 May 7/4: Flapper Dictionary flipper – A male Flapper.

3. (Aus.) a girlfriend.

1912

Sun. Times (Perth, Aus.) 25 Feb. 14/2: They called their girlfriends tarts and flippers.

4. a euph. for fucker n. (3)

1956		‘Charles Raven’ Und. Nights 18: Flip you, you flipper.
1956		J. Reynolds Woman of Bangkok (1959) 31: Some sixth sense told me that flipper would stop for me.

5. a friend; in general, a man.

1921		T. Thursday ‘Hail the Professor’ in Top-Notch 1 Sept. 🌐 When two wise flippers meet they generally part with the same suit of clothes, watch, and ideas—splitting the bunk fifty-fifty.
1944		T. Thursday ‘Base on Balds’ in Sports Winners Spring 🌐 Among the weird and peculiar sights [...] are no less than four flippers who are as bald as both sides of a dime.

6. (Aus.) a fool.

1983

R. Aven-Bray Ridgey-Didge Oz Jack Lang 28: Flipper Fool.