What is the origin of the idiom "behind one's back" and when to use it?

The origin of the idiom "behind one's back" is not attributed to a specific historical event or source but rather emerges from the metaphorical association of a person's back with their unawareness or lack of visibility. The phrase draws upon the idea that when someone's back is turned or facing away, they cannot see what is happening behind them. It is commonly used when discussing interpersonal relationships, workplace dynamics, or social interactions.