Cherchez la femme
The translation from the French is "look for/seek the woman". It is used when a man behaves unusually or gets into a quarrel or other difficulty and the reason for it is sought.
'Cherchez la femme' is sometimes mistakenly thought to refer to men's attempts to pursue romantic liaisons with women. In fact, the phrase, which is occasionally used in its loose English translation 'look for the woman', expresses the idea that the source of any given problem involving a man is liable to be a woman. That isn't to say that the woman herself was necessarily the direct cause of the problem, as in Shakespeare's Macbeth for instance, but that a man has behaved stupidly or out of character in order to impress a woman or gain her favour.
The expression was coined by Alexandre Dumas (père) in the novel The Mohicans of Paris, 1864, in the form of 'cherchons la femme'. In John Latey's 1878 English translation, Dumas' detective, Monsieur Jackal, concludes that a woman must have been involved in the crime being investigated:
"Where's the woman? Seek her."
His opinion was later confirmed by a colleague:
"Ah! Monsieur Jackal, you were right when you said, 'Seek the woman.'"
The phrase was adopted into everyday English use and crossed the Atlantic by 1909. It was well enough known there by that date for O. Henry (William Sydney Porter) to use it as the title of a story - Cherchez La Femme, which includes this line:
"Ah! yes, I know most time when those men lose money you say 'Cherchez la femme' - there is somewhere the woman."
Dumas was, of course, the author of many popular novels, including The Count of Monte-Cristo, 1844, from which he earned a sizeable fortune. He had a bash at following in the footsteps of his eponymous hero when he had the lavish Château de Monte-Cristo built in 1846. Life copied art also in his ruinous attempts to attract women to the high life at the chateau. When biographers looked to see where all his money went, the only explanation needed was 'cherchez les femmes'.
See also - other French phrases in English.