That's not cricket

Cricket has long carried a reputation in England as the model of fair play and gentlemanly conduct, governed by an unwritten code of sportsmanship as much as its formal rules. By the 19th century, "it isn't cricket" was already being used to describe behaviour, on or off the pitch, that violated that sense of fairness, extending the game's own ethos into a general-purpose idiom for unsporting or underhand conduct.