thick as thieves
Intimate, closely allied, as in The sisters-in-law are thick as thieves. This term uses thick in the sense of “intimate,” a usage that is obsolete except in this simile. [Early 1800s]
Words nearby thick as thieves
thibet,
thibodaux,
thick,
thick and fast,
thick and thin,
thick as thieves,
thick client,
thick milk,
thick register,
thick skin,
thick-knee