Accusative cases, the headword one evidently quoted from somewhere. Likewise or similarly in other lexica; references at Photius sigma776, where Theodoridis regards the transmitted headword suneu/nion as an error for su/neunon. Adler had already drawn attention to Sophocles, Ajax 1301, where cu/neunon occurs; Theodoridis adds Pindar, Olympians 1.88, which has su/neunon itself; and [DW] see also e.g. (?)Aeschylus, Prometheus Bound 866.
cf. sigma 1519.