*fire under someone
Fig. something that makes someone start doing something. (*Typically: build ~; light ~; start ~.) The teacher built a fire under the students, and they really started working. You had better light a fire under your staff. Either that or we will lay off some of them.
See also: fire
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.