1. light a fire under - Verb
2. light a fire under - Phrase
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.
Let me see if I can light a fire under the waiter to get our order sooner.
Every writing teacher I ever had except for one told me I was an awful writer, had no idea what I was doing, and should stop immediately. It only took the one to tell me something different to light a fire under me. Catherynne M. Valente