Bladder : nf A bladder is a musculo-membranous reservoir in which urine accumulates which arrives from the kidneys through the ureters.
To mistake bladders for lanterns: to be mistaken in an absurd and naive way.
In its current form, this expression is attested in the XNUMXth century, but the Robert of the expressions Rey and Chantreau give the older forms: "sell bladder for lantern" from the XNUMXth century, then make lantern bladders. And there are at least two schools as to its origin: the first by these pig (like beef) bladders which were sometimes used as emergency lanterns, once a lit candle was placed in it. Suddenly, it was easy to make the passing simpleton believe that such a bladder hanging from the ceiling was a lantern, because of their similarity in shape.
The second school juxtaposes the word lantern used formerly in the plural, in the sense of nonsense, absurdities and bladder in the sense "to sell bladder" which meant to sell wind, because of the air which inflates said bladder, enveloping very little. value.