[māo shǔ tóng chǔ]   

猫    鼠    同    处    比喻官吏失职，包庇下属干坏事。也比喻上下狼狈为奸。同“猫鼠同眠”。

猫鼠同处，作宾语、定语；比喻上下狼狈为奸

Thieves and police work together, as the cat and the rat sleep together

猫鼠同眠    猫噬鹦鹉    猫哭老鼠    猫鼠同处    猫鼠同乳