Definition
To be in charge and make all the important decisions within a group or organization.
当家作主
Since his promotion, he has been ruling the roost in the office.
自從他升職後，他一直在辦公室裡當家作主。
In their family, it's usually the mother who rules the roost.
在他們家裡，通常是母親當家作主。
Don't be fooled by his young age; he rules the roost in our group.
別被他年輕的外表騙了；他在我們群體中當家作主。