Common in Gaulish onomastics (cf. Stüber 2005: 71 f., Stüber et al. 2009: 259). The origin of the suffix in Celtic is unclear. Greek has a motion suffix (Ionic) -ισσα- < *-ik-i̯a- (see Lühr & Balles 2008: 236), which made its way into Old Irish (-es) via late Latin (Stüber 2006: 227 f.), but this late loan is unlikely to have a connection with Gaulish -iss-. An earlier loan directly from Greek cannot be excluded, but the existence of -uss- besides -iss- indicates that the suffix is inherited (cf. -il(l)-/-ul(l)-). Cf. Delamarre 2010: 70–74, who analyses a number of names with dental clusters including ss as compound with second element *-sth₂-o- 'stand, be situated', arguing that the latter developed into a suffixoid -sto-; this is a potential source for a resegmented onomastic suffix, though the issue ties in with the difficult chronology of the development of tau gallicum.
|Delamarre 2010||Xavier Delamarre, "Notes d’onomastique vieille-celtique", Keltische Forschungen 5 (2010), 49–87.|