So I did the once for all thing: I called my French friend Ginette at Niort, France. The meaning is what it seemed to be from the beginning , but the genuine French expression, no longer used in today's speach is: "Mentir comme un arracheur de dents". Translation: "To lie like a tooth jerker"
It dates from the days when the word dentiste was not yet in use.

I'm not sure if today's dentists are completely free from deceiving actions like their free style predecessors. Sometimes advising expensive treatments which are maybe not that
necessairy.
(hope not to offensive possible dentists at the board, who of course would never be like that).