means A person you don't know — someone outside your circle of acquaintance, whether they're a passing face on the street or a newcomer to a place.

from From Old French 'estrangier' (foreign, alien), which grew from Latin 'extraneus' meaning 'external, from outside' — itself rooted in 'extra,' 'outside.' The same Latin source quietly fathered 'strange' and 'extraneous,' so a stranger is, at heart, simply someone from the outside not yet let in.