Likewise or similarly in other lexica (references at Photius omicron287 Theodoridis), including Hesychius omicron747, where Latte claims the source of the headword as Euripides, Hecuba 60 (feminine accusative there).
Pollux 3.82 mentions the view that su/ndoulos (the present gloss) means, more precisely, 'slave of the same master'. See LSJ under su/ndoulos and cf. sigma 1462.