Light a fire under (someone)
Meanings of "Light a fire under ..."
To stimulate, motivate someone to work or act more quickly or enthusiastically. Also there is 'light a fire under someone's ass/arse/butt'. This is a work environment expression often, but is also good for home use too.
To light a fire under something, the thing being a mule, a donkey was an often said remedy for one that did not want to get up and work/move. Fire gave them the initiative to move quickly.
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