'ET' originates from the abbreviation of 'extraterrestrial,' where 'extra-' meant 'outside' and 'terrestrial' meant 'of the earth.'

'Extraterrestrial' was shortened to 'ET' in popular culture, especially after the release of the film 'E.T. the Extra-Terrestrial.'

Initially, it referred to anything outside the earth, but now it is commonly used to refer to alien beings.