Region: US | Usage example: Before announcing the offer, I told him to curb his enthusiasm.

Curb your enthusiasm

Meaning

Limit your excitement until the outcome is assured.

Warns against premature celebration.

Origin notes can vary by region and source; this page gives the commonly reported explanation and usage context.

Driving metaphor for slowing down momentum.

Became more widespread after the 1990s sitcom of the same name.