Light a fire under someone's behind

Meaning

This means to motivate someone to act quickly, usually because they have been lazy or procrastinating. It is a colorful way to describe applying pressure.

Stern, bossy

Used in a sentence

• You better light a fire under his behind if you want that garage cleaned before the party.
• The project deadline is Friday, so we need to light a fire under the contractor.
• I had to light a fire under my teenager to get those college applications submitted.