Don't assign the duty of protecting or controlling valuable information or resources to someone who is likely to exploit that opportunity. You're going to put your ex-convict brother-in-law in charge of your business? I can't tell you how to run your company, but don't let the fox guard the henhouse.

A person likely to exploit the information or resources that they have been charged to protect or control. My sister is going to put her ex-convict brother-in-law in charge of her business, and I'm worried he'll be like a fox guarding the henhouse.

The only male in an all-female environment. Getting a male teacher is a big deal when you go to an all-girls school! Finally, a cock in the henhouse! My grandpa always complains that he's a cock in the henhouse living here with my grandma, my mom, and my twin sister.

Someone with bad intentions. (A fox would prey upon hens in a henhouse.) I'd watch out for him if I were you—he walks around here like a fox in the henhouse.