Argon

The existence of argon in air was suspected by Cavendish in 1785, as there was an unknown residual gas remaining, when oxygen and nitrogen were removed from air. The element wasn't discovered until 1894, by the Scottish chemist William Ramsay and the English physicist Robert John Strutt (Lord Rayleigh), in liquefied air [2, 11].

From the Greek word argos meaning lazy/inactive [11].