The French phrase cherchez la femme, search for the woman, is used to indicate that the key to a problem or mystery is a woman, and that she need only be found for the matter to be solved.

It first appeared as a catchphrase used by M. Jackal, a police detective, in Les Mohicans de Paris, a novel written from 1854 to 1859 by the French novelist and dramatist Alexandre Dumas père (1802-70). The following is a dialogue between M. Salvator and M. Jackal:

     (volume 1 – 1863 edition)
– De quoi s’agit-il, en deux mots ?
– D’un enlèvement.
– Cherchez la femme !
– Parbleu ! c’est ce que nous cherchons.
– Oh ! non, pas la femme enlevée.
– Laquelle, alors ?
– Celle qui a fait enlever l’autre.
– Vous croyez qu’il y a une femme là dedans ?
– Il y a une femme dans tout, M. Salvator.
     translation:
– What is it, in brief?
– An abduction.
– Search for the woman!
– By Jove! that’s what we are searching for.
– Oh! no, not the abducted woman.
– Which one, then?
– The one who had the other abducted.
– You believe there is a woman in that?
– There is a woman in everything, M. Salvator.

The narrator later explains, about M. Jackal:

Toutes les fois que l’on venait lui dénoncer une conspiration, un assassinat, un vol, un enlèvement, une escalade, un sacrilège, un suicide, il ne faisait qu’une réponse : « Cherchez la femme ! »
     translation:
Every time someone came and denounced to him a conspiracy, a murder, a theft, an abduction, a burglary, a sacrilege, a suicide, he made only one answer: “Search for the woman!”

There is therefore an error in the 1972 Supplement to the Oxford English Dictionary, which indicates that Dumas uses the phrase only “in the form cherchons la femme” (= “let’s search for the woman”).

I have found a use of cherchez la femme, in The Daily Telegraph (London) of 20^th April 1870, which predates by 23 years its earliest recorded instance in an English text:

One cherished hope we have—it is perhaps Utopian, but we will not entirely give it up—[is] that, some day, somewhere, in some county or borough prison, may be seen an eminent solicitor wearing the prison dress, with “Imprisoned for election bribery” recorded against his name. For, while the French have a saying as regards all quarrels, “Cherchez la femme,” we may always say, as regards any corrupt transaction in electoral politics, “Find out the attorney.”

The French phrase is similar to lady in the case, an expression used since the 17^th century to indicate that a woman is the cause of a situation or the key to solving a problem. For example, in The Hare and many Friends, from Fables. Invented for the amusement of His Highness William Duke of Cumberland (1727), the English poet and dramatist John Gay (1685-1732) writes how the bull, from whom the hare has asked for help, excuses himself:

(1767 edition)
Love calls me hence; a fav’rite cow
Expects me near yon barley mow:
And when a lady’s in the case,
You know, all other things give place.