As far as I know "not cricket" means "unfair, unsportsmanlike". Please kindly confirm if this is the case in the following context, taken from "1919" by John Dos Passos:
Her brother George turned up at the office once day with a captain's two silver bars on his shoulders. His whipcord uniform fitted like a glove, his puttees shone and he wore spurs... He was going to Cambridge for the spring term and called everybody blighters or rotters and said the food at the restaurant where Eveline took him for lunch was simply ripping. After he'd left her, saying her ideas were not cricket, she burst out crying.