indirect pupil; pupil of one's own pupil
Refers to a student taught by one's own student, i.e., a grand-pupil in a master-disciple lineage. Common in traditional arts, crafts, or martial arts contexts.
He is an indirect pupil of mine, being my own student's student.
茶道の世界では、又弟子も大切に指導する。
In the world of tea ceremony, even indirect pupils are carefully guided.
孫弟子 is a more common synonym for 'indirect pupil' or 'grand-pupil'. 又弟子 is less frequent but carries the same meaning.
Compound of 又 (mata, 'again; indirect') and 弟子 (deshi, 'pupil'). The reading またでし is a straightforward combination of the two components.