light a fire under
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.
ADJ.
big, dark, fat, heavy, light, new, proud, sure
The heavy box under the stairs has been there since we moved in.
VERB + UNDER
accept, act, appeared, band, books, bridge, cash, coming
The refugees had to accept difficult conditions under the bridge while waiting for shelter.
UNDER + NOUN
baton, condition, currents, desk, discussion, leadership, lee, mattress
The project is currently under discussion between the two departments.
PREP.
down, from, on
The old bridge had water flowing under it from the mountain stream.
ADV.
marvellously, only
The old bridge held up marvellously well even though it had been underwater for years.