- What is the origin of the word earthling?
- Are there other words with a similar meaning (marsling, venusling)?
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As reported on Etymonline:
However, I found this too:
The word earthling is formed from the word earth and the suffix -ling, meaning something from earth, or something related to earth. Here earth can either refer to the planet, or to the soil.
It's now mostly used in science fiction to mean humans from earth, but it existed even before that. One older use is in the meaning of farmer or ploughman, another is in the meaning of mortal as contrast to heavenly creatures or gods.
In science fiction the term Terran is also commonly used.
NOAD gives for “-ling”:
This gives you some more examples...
'Earthling', before the sci-fi era, had somewhat the same connotations as 'troglodyte', an lesser, earthy person (see the etymonline provenance as in mplungjan's answer). This is what probably inspired Heinlein to use it to have people not from Earth refer to people from Earth. The sci-fi literature has created a number of alternatives, like 'earther' and 'terran'. 'Earthling' has too much of a 1950's scifi connotation and is not used in contemporary sci-fi, except for pejoratively.
As to other planets, generally recognized inhabitant names, though having only imaginary referents, are:
'Mercurial','venereal', 'martial', jovial, and 'saturnine' are the respective metaphorical versions (that is, not referring to an inhabitant, but to 'slippery/fickle', 'sex-related', 'military-like', 'convivial', and 'gloomy', respectively). 'Earthy' would be the closest analog for Earth.
The other planets don't really have commonly recognized inhabitant names, but they can be easily created anew: Mercutian, Uranian, Neptunian, Plutonian (yes, I know... then Erisian, Cerean, Haumean, Makemakean...).