A fellow student or schoolmate; someone who studied together with another person at the same institution.
From Latin 'com-' (together) + 'pupillus' (student, ward, or young person). The Latin 'pupillus' originally meant ward or orphan under guardianship before extending to students.
This lovely word has almost completely vanished from English, but it once described a real social bond—students who learned together were 'compupils,' a term suggesting shared intellectual journey, not just coincidental school attendance.
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