Many a true word is spoken in jest is an adage, aphorism or proverb.
A version of this appears in the Prologue to "The Cook's Tale" (written in 1390) by Geoffrey Chaucer: "Ful ofte in game a sooth I have herd saye!". [1]
An early print appearance of the most familiar form of this aphorism was in Volume VII of the Roxburghe Ballads , where it appears in the prologue to The Merry Man's Resolution, or A London Frollick. The ballad purportedly goes back to the 17th century, but the introductory verse was probably written by the editor of the collection, Joseph Woodfall Ebsworth: [2]
He goes a wooing, yet the matter's so,
He cares not much whether he speeds or no;
'Cause City Wives and Wenches are so common,
He thinks it hard to find an honest woman.
Be n't angry with this fellow, I protest
That many a true word hath been spoke in jest.
By degrees he layes a wager, money's scant,
Until five shillings out; then ends his Rant.
James Joyce combined this sentiment with the similar adage of in vino veritas to coin the phrase in risu veritas (in laughter, truth). [3]