Definition
A minor or secondary part of something which is controlling or dominating the whole or the main part.
Origins
First used in the 1858 play Our American Cousin. The shortened form wag the dog, with a political sense, was popularized by the film Wag the Dog (1997). For the verb phrase, see the tail wags the dog.
See Also
How People Actually Use It
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."
Source: Urban Dictionary