a fire under (someone or something)

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a fire under (someone or something)

The motivation to take action. Typically used in phrases like "light a fire under (someone or something)" and "start a fire under (someone or something)." The deadline really lit a fire under us—we never would have gotten all that work done otherwise. Something must have lit a fire under Jenna—she has really started taking her schoolwork seriously.
See also: fire