mean business
mean business {v. phr.}, {informal}
To decide strongly to do what you plan to do; really mean it; be serious.
The boss said he would fire us if we didn't work harder and he means business.
When she went to college to study, she meant business.
He just liked the company of the other girls he dated, but this time he seems to mean business.
Categories:
Source: A Dictionary of American Idioms