1
general::
Phrase(s): light a fire under someone
Go to a fire under someone.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, build a fire under. Urge or goad to action, as in If we don't light a fire under that committee, they'll never do any work. This hyperbolic colloquialism uses light in the sense of "ignite," a usage dating from the mid1100s.
American Heritage Idioms