It means that it is not our property, that it does not belong to us. That it belongs to another, that belongs to another.
That it is strange, that it is not one's own, close or known. It comes from the Latin alienus, a, um ("stranger, foreigner"), from alius, a, ud ("alien, other"), which is borrowed from Greek 945; 955; 955; 959; 962; ( allos "other" ) .
Alien: Property which does not belong to us.