answer:The often used phrase “little green men” to refer to aliens may have come from Burroughs’s first book on Mars, “A Princess on Mars.” “A Princess on Mars” (written in 1911, first published hardback in 1917). He describes the “green men of Mars,” and they reappear frequently in his other Martian novels. Still, he never uses the exact phrase. The Oxford English Dictionary’s first reference for “little green man” is from Kipling’s “Puck of Pook’s Hill” (1906). Its use here refers to an actual person who has been tattooed green, and so although it coins the phrase “little green man,” it does not appear to be a reference to aliens. The next OED reference to “little green men” is not until 1961, from Partridge’s Dictionary of Slang: “Little green men, mysterious beings alleged to have been seen emerging from flying saucers.”