means The act of giving someone a vaccine — a preparation that trains the immune system to recognize and resist a specific disease without causing the full illness.

from From Latin vacca, 'cow' — a genuinely bovine origin. In the late 18th century, the physician Edward Jenner noticed that milkmaids who caught the mild cowpox seemed spared from deadly smallpox, and he used material from cowpox sores to inoculate against it. The cow-derived procedure was dubbed 'vaccination,' from variolae vaccinae, 'cowpox.' Later the word was generalized to cover all such immunizations, so even your flu shot still carries a faint echo of a dairy barn.