rule of three, n. — Green’s Dictionary of Slang

rule of three n.

[SE rule + three, i.e. the penis and testicles; note Williams: ‘This alludes to the golden rule whereby a fourth number is deduced from a given three numbers’]

1. the male genitals.

1720		in D’Urfey Pills to Purge Melancholy VI 329: You Dames that are Wed who can make it appear, / That you lose an Estate for want of an Heir; / This Accountant will come with e’er a Fee, / An warrants a Boy by his Rule of Three.
1790		‘Bumper Allnight. Esquire’ Honest Fellow 73: You ladies that are with a husband unblest, / And are minded to make him a delicate beast,/ He’ll fix the brow antlers just where they shoul be, / And all by his art in the sure rule of three.
1840		‘The Rule of Three’ in Facetious Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 262: O the charming Rule of Three! / O the charming Rule of Three! / While I have life I still shall love, / And bless the charming Rule of Three.

2. sexual intercourse.

1937–84

Partridge DSUE (1984) 996/1: C.19–20.