a shot in the arm

a Determinative

— A is used when the following word could be any of a certain type. Compare "A book I saw on the shelf" and "The book I gave you yesterday".

a Noun

— A is the first letter of the alphabet. The letter "a" comes before "b". — In some schools, an A is a very high grade. Ron got an A on his earth science test.

shot Noun

— The sound of a gun firing. — Small balls or pellets of lead typically fired from a shotgun.

in Preposition

— Used to show that something is inside something else. The cat is in the box. — Used to show that someone is at home, or is available. Is John in? The Doctor is now in. — Used to show movement towards the inside. The rain came in through the window.

the Determiner

— Used, instead of a, to reference something specific, already known to exist. Compare "I read a book." and "I read the book." — Used with a stress, to show that the word following is special. Are you the John Smith that I went to school with? — Used with an adjective that acts like a noun to mean all of the people concerned The poor are always with us. — Used with superlatives forms of adjectives and adverbs. You are the best.

arm Noun

— One of the upper limbs people have; arms start at the shoulder and go down to the wrist, sometimes the hand is included. — A weapon.

arm Verb

— To give or sell weapons to a person, group, or country. Our taxes pay to arm the military. — To get a tool or a weapon ready for action; to activate. Remember to arm the alarm system.