From 1785, Henry Cavendish (1731-1810) suspected the presence of Argon in air. But it was not until 1894 that Argon was officially discovered by Sir William Ramsay (1852-1916) and Lord Rayleigh (1842-1919).
This discovery was the result of work on Nitrogen synthesis. Until the 19th century, two types of Nitrogen were recognized:
NH₄NO₂ → N₂ + 2H₂O
Ramsay and Rayleigh noticed that the density of atmospheric Nitrogen was higher than that of chemical Nitrogen. They inferred that air Nitrogen was not pure, and that it was made of some other component they tried to identify.
They succeeded in isolating this new element, and a spectroscopic analysis confirmed that a new and unknown compound was indeed involved. They also isolated two other new gases: Neon and xenon.
Ramsay suggested the name Argon, from the Greek a, “private”, and ergon, “work”. Since it is not very reactive, Argon is a “lazy” gas that “does not work”.
Today, Argon’s symbol is Ar; but until 1957, it was simply A.