shot in the arm, a - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]