This expression derives from the invigorating effect of injecting 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.”
Getting it in the ARM!
Roll up your Sleeve!
I Don't need a SHOT!
Right in the ARM!
You have lots of Fatty MUSCLE!
Skinny Girl gets her SHOT in the ARM! OW!
Muscle Girl making a face when getting her SHOTS! nice strong arms by the way.
In her Arm it goes! OW! OW! OW!
When she was called to see the Nurse, She never had a clue this was going to happen to her!
Intramuscular (or IM) injection is the injection of a substance directly into a muscle. In medicine, it is one of several alternative methods for the administration of medications (see route of administration). It is used for particular forms of medication that are administered in small amounts. Depending on the chemical properties of the drug, the medication may either be absorbed fairly quickly or more gradually. Intramuscular injections are often given in the deltoid, vastus lateralis, ventrogluteal and dorsogluteal muscles.