When a person allows their decisions to be dominated by someone who is clearly unfit for the role, rather than being responsible and taking control of the situation.
When parents let their children make the rules, instead of the other way around, is a prime example of the tail wagging the dog.
The need to defecate, make a doody, make a number two,to poop
Having finished my meal, I turned to my friends and said, I really must be wagging the brown tail.After a meal of that scope I had to wag the brown tail, almost immediately.Ron ran to the bathroom clutching his buttocks-shreiking "I need to wag the brown tail!!!"