Cherchez la femme is a French phrase which literally means "look for the woman." The implication is that a man behaves out of character or in an otherwise inexplicable manner because he is trying to cover up an affair with a woman, or trying to impress or gain favor with a woman.
The expression comes from the 1854 novel The Mohicans of Paris by Alexandre Dumas (père). The first use in the novel reads:
Cherchez la femme, pardieu ! cherchez la femme !
The phrase is repeated several times in the novel. Dumas also used the phrase in his 1864 theatrical adaptation, which reads:
Il y a une femme dans toutes les affaires ; aussitôt qu'on me fait un rapport, je dis : « Cherchez la femme ! »
Translated into English this reads:
There is a woman in every case; as soon as they bring me a report, I say, 'Look for the woman!'
The phrase embodies a cliché of detective pulp fiction: no matter what the problem, a woman is often the root cause. The phrase has come to refer to explanations that automatically find the same root cause, no matter the specifics of the problem.