She's a student from a poor family and has worked hard since she was young.
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Literary and slightly formal. Common in phrases like 寒门学子 (student from a poor family) and the widely debated saying 寒门难出贵子 (it's hard for poor families to produce successful children).
history
Originally referred to families outside the aristocratic clans of medieval China who were blocked from official posts by the 门阀 system. Modern usage echoes that sense of structural disadvantage.
