To rein in is to pull a horse up, or to hold something back. Rein in the spending.
The literal sense is pulling on the reins, and almost every use now is the other one. To rein in a budget or a colleague is the commonest way the phrase is heard.
1phraserein inTo pull a horse up, or to hold something back.
“The board tried to rein in the whole department.”
rein in is an action.
Practised in Horses & Riding