Green’s Dictionary of Slang

1. a penis.

1902		D. St John Memoirs of Madge Buford 70: Holding Ralph’s darling diddler in one hand [etc].
1980		Maledicta IV:2 (Winter) 190: Diddler is [...] that which is used to diddle (= titillate) the diddley-pout = vagina.

2. a male homosexual.

c.1864

parody in Rakish Rhymer (1917) 122: Our bums are getting hot, / Our shifts are going to pot, / For we’ve no money and no thread to mend them, / O, what will a poor diddler do ?

3. (US) a performer of sexual intercourse.

1893		‘Bob Sterling’ Town-Bull 9: She sighed and wiggled, called me a splendid diddler.
1902		D. St John Memoirs of Madge Buford 55: ‘Suck every drop out with your puckering cunt, my pretty diddler!’.

4. a masturbator.

1934		‘J.M. Hall’ Anecdota Americana II 111: The impotent diddler / Finds life’s greatest thrill / Just catching the scents.
1950		see sense 4.
1972		(ref. to 1930s) B. Rodgers Queens’ Vernacular 116: a chronic masturbator [...] diddler (’30s).

5. (US/Can.Und., also kiddie diddler) a child molester.

1950		Goldin et al. DAUL 58/1: Diddler. 1. One who impairs the morals of minors; a sex degenerate. 2. A masturbator.
1978		R. Caron Go-Boy! 148: beware! Stoolies, Diddlers and Turkeys; Enter at your own risk!
1989		J. Morton Lowspeak 49: Diddler – sex offender.
1992		Bentley & Corbett Prison Sl. 35: Diddler also Short Eyes An inmate who is in prison for child molestation.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Diddler: Child molester. (CT).
2013		J. Stahl Bad Sex on Speed 29: God made a lower place in hell for lowlife drug thieves than kiddie diddlers.