>You lie like a dentist at a fair.

This expression is used by one of the characters in the English translation of "The Age of Reason" (1945) by Jean-Paul Sartre, translated by Eric Sutton (1962).

It gets zero Google-hits.

Maybe it's a literal translation of a French idiom. Any ideas as to why French dentists at fairs might embody the quiddity of dishonesty?