cf. already Hesychius sigma2595, glossing with sunqh/kh, o(/rkoi, sumbo/laia "covenant, oaths, contracts." These entries are probably generated by Homer, Iliad 22.261, where the accusative plural occurs; see also Apollonius Rhodius, Argonautica 3.1105 and (dative plural) 1.300.
See the related noun sunh/mwn at sigma 1561.