the.com/alien
Anything sufficiently other to make you nervous, from Mars or from across the border.
means A being or thing belonging to somewhere else — another country, another planet, or another category entirely.
from From Latin alienus, belonging to another, from alius, other; the same root that gives us alias and alibi, all flavors of elsewhere.
Legal sensePredates the sci-fi sense by centuries.
Verb formTo alienate is to make something other.
Pop cultureThe space monster meaning exploded only post-1950s.