shot in the arm, a
A stimulus or booster, something vitalizing or encouraging, as in Getting a new concertmaster was a real shot in the arm for the orchestra. This colloquial expression alludes to a stimulant given by injection. [c. 1920]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.