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A shot in the arm

Meaning

A stimulus.

Origin

From the invigorating effect of an injection of drugs. A shot is of course US slang for an injection, either of a narcotic or medicinal drug. That term has been in use since around the beginning of the 20th century, for example, this piece from the San Francisco Chronicle Supplement, October 1904:

"I varied hardly a minute each day in the time of taking my injection. My first shot was when I awoke in the morning."

'A shot in the arm' came soon afterwards and the first mention of it in print that I can find is from the Wisconsin newspaper The Capital Times, December 1920:

"California is a 2 to 1 favorite out there on press dope. A shot in the arm please. Dope is right. Now I hear the little birdies singing."

Dope is another US slang term from around the same date. It was coined in the late 19th century to mean 'drug or strong drink' and, as an independent meaning, to mean 'horse racing information' and, later, any form of inside information. The Capital Times report above punningly uses both of those meanings.