Same entry in Photius and other lexica. (Earlier, Hesychius had had a similar one for e)si/neto.) The headword is third person plural, imperfect, of si/nomai. It must be quoted from somewhere; Theodoridis on Photius s.v. suggests Herodotus 5.74.2, but there are two other possibilities in that author alone, not to mention others elsewhere.
cf. sigma 462.