In ancient times silk cloth as a whole was called 'zeng'.
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history
缯 is not used as a standalone word in modern Chinese; it appears only in classical texts and surnames. The silk radical 纟 marks the fabric meaning while 曾 supplies the sound, exactly as in 增 and 赠.
register
Archaic and literary. Read 缯 as a surname in some contexts; otherwise it is confined to old texts and etymology notes.