light a fire under

== English == === Etymology === 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. === Pronunciation === === Verb === 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) (idiomatic, transitive) To motivate or encourage (someone) to start sooner or move faster.