cover of Thrilling Wonder Stories (August 1951)
The noun earthling is composed of earth and the suffix -ling, meaning, in this case, a person belonging to.
In science fiction, it is used by aliens to refer to an inhabitant of the earth.
But this noun, which dates back to the late 16th century, originally denoted an inhabitant of the earth as opposed to heaven. It is first attested in Christs teares ouer Ierusalem (1593), by the English pamphleteer Thomas Nashe (1567-circa 1601):
Wilt thou ratifidely [= positively] affirme, that God is no God, because (like a Noune substantiue) thou canst not essentially see him, feele him, or heare him. Is a Monarch no Monarch, because hee reareth not his resiant [= resident] throne amongst his vtmost subiects? Wee (of all earthlings) are Gods vtmost subiects, the last (in a manner) that he brought to his obedience: shal we then forget that wee are any subiects of his, because (as amongst his Angels) he is not visibly conuersant amongst vs?
This noun also denoted a worldly or materialistic person. In Of the obseruation, and vse of things (1600-01), the English essayist William Cornwallis (circa 1579-1614) wrote:
I haue not beene ashamed to aduenture mine eares with a ballad-singer, and they haue come home loaden to my liking, doubly satisfied, with profit, & with recreatiō. The profit, to see earthlings satisfied with such course [= coarse] stuffe, to heare vice rebuked, and to see the power of Vertue that pierceth the head of such a base Historian, and vile Auditory.
The recreation to see how thoroughly the standers by are affected, what strange gestures come from them, what strained stuffe from their Poet, what shift they make to stand to heare, what extremities he is driuen to for Rime, how they aduenture their purses, he his wits, how well both their paines are recompenced, they with a silthy noise, hee with a base reward.
Before being used in science fiction, earthling denoted a person who lives on the earth as opposed to another planet in the following from The Fayetteville Observer (North Carolina) of 18 October 1858 (cited by the Oxford English Dictionary – 3rd edition, 2010):
The last time the great comet was seen by us earthlings is said to be three hundred years ago.
But the word has been used in other senses. For example, the author of an article published in The Manchester Courier (Lancashire) of 5th August 1907 wrote, about the advantages of taking a trip in a balloon during the holiday season:
No casual acquaintance nor tripper can suddenly haunt you and force on you the platitudes that propriety and good breeding extract on shortest notice from the anchored earthling.
Similarly, a correspondent who had been the passenger of a bomber “equipped with the amazing automatic pilot” published the following in The Northern Whig (Antrim) of 14th August 1930:
I have flown scores of thousands of miles in the daylight, including a flight to Cape-town and back, but this first experience of night flying brought a new charm of complete isolation. In the night with only a mystery of grey skies and dimly seen ground it is easier to forget that you are an earthling.
And, in What Disobedience did for Bobbin, a fairy story published in The Lichfield Mercury (Staffordshire) of 24th December 1926, earthling was used in contrast to elf.