Literal Translation
To sell the lock of hair/fuse/wick
Actual Meaning
To give the game away, spill the beans, tell a secret
Etymology
We’re talking about the fuse that was once used to light a flame with an oil lamp, the kind that allowed pyrotechnicians to detonate mines or fire artillery pieces, or the kind that is still used to light firecrackers (the kind that explode, not the kind that are smoked).
To understand the origin of this expression, we must go back to the 16th century, when the expression ‘éventer ou découvrir la mèche’ (to fan or uncover the fuse) was used. When a pyrotechnician fanned (exposed to the air) or uncovered the fuse of an enemy mine or explosive device, he was able to prevent damage.
The verb “éventer” first meant ‘to expose to the wind, to the air’, then ‘to spread rumours, to divulge’, before finally taking on the meaning of ‘to find, to discover’. Metaphorically, “éventer la mèche” came to mean “to uncover the details of a plot (before it causes damage) or a matter that should remain hidden”.
Then, from the 19th century onwards, the word “vendre” (to sell) in the sense of “to betray” (‘you’re nothing but a sell-out!’) was added to the original expression to give us the one we use today.