light a fire under

Referring to a time when, if chimney sweeps were afraid to ascend a chimney, the fire would be lit under them in order to motivate them to climb to the top.

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light a fire under (third-person singular simple present lights a fire under, present participle lighting a fire under, simple past and past participle lit a fire under)

1. (idiomatic, transitive) To motivate or encourage (someone) to start sooner or move faster.

   Let me see if I can light a fire under the waiter to get our order sooner.