How did 背后 come to mean "behind one's back" both literally and figuratively, like in English?
背后(behind) is a compound word made up of [背 = back/ back side (n)] + [后 = behind (preposition)]

背 in 背后 can refer to &quot;the back&quot; (背脊) of a person
e.g. 在你背后批评你
背 here refers to your back (背脊)
Although 背 here literally mean one's back, it is used as a metaphor for &quot;place you cannot see = instances unknown to you &quot;

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背 in 背后 can also refer to &quot;the back side&quot;(背面) of an object
e.g. 在门背后
背 here refers to the back side of the door (门背面)
In this case, the meaning of &quot;背&quot; (the back side) is completely literal