Same or similar entry in other lexica (and various scholia; references at Photius sigma719 Theodoridis).
Despite this glossing, the headword noun only usually -- not invariably -- means 'unlucky chance circumstance': cf. sigma 1408, sigma 1409, and see generally LSJ s.v. (Words beginning with syn- denote the coincidence of two or more events. The root for 'chance, fortune' tych- originally denoted hitting a target, the opposites missing a target, but quickly referred to pure chance.)