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does not help us to decide the orig. forms, as intervocalic ‑s- simply drops in Ir. The combination goes back beyond Prim. Kelt.; in Lat. it is a proper compound: mītigāre, rēmigāre, nāvigāre, etc.

This is the Ar. denominative suff. *‑i̯e‑, as seen in Skr. apas-yá-ti ‘is active’ from ápas- ‘act’; Gk. τελείω (< *τελεσ-ι̯ω) from τέλεσ-; δηλόω (< *; etc., Meillet, Intr.² 183.

In old formations the ‑i̯- of course affects the preceding vowel in W.; thus niweidiaf ‘I injure’: niwed, Ml. W. er-nywed w. 48a, § 76 iv (4); peidi̯af, v.n. peidio ‘to cease, be quiet’ < Brit. *pat- < *q^u̯(i̯)ə‑t‑, √q^u̯ei̯ē‑; Lat. quiēs.

In W. the suff. is not added to adjectives. But ‑ai‑, Ml. ‑ei- in the ult. may be caused by the i̯ of the lost adjectival ending *‑i̯os; and the i̯ is kept in the vb.; thus disglair ‘bright’ < *dē-eks-kl(i̯)ari̯ós (: claer < *klii̯arós § 75 vi (1)); hence W. disgleiriaf ‘I shine’, v.n. disgleiri̯o. From these forms it was extended to other adjectives with ‑ei- as perffeithio, perffeiẟẏaw from perffeith < Lat. perfectus; and with ‑i̯‑, as gwiri̯o ‘to verify’ from gwir § 35 iii (but cywiro from cywir).

The suffix is generally used in verbs borrowed from Eng.; thus pasi̯af ‘I pass’, passiodd Can. ii 11; peintẏaw ‘to paint’ (§ 16 v (2)); ystopẏawẟ s.g. 72 ‘stopped’. In some cases two forms are used; thus ffaelẏaw s.g. 285 ‘to fail’, beside ffaelu do. 348, the latter being the treatment of native words with ‑ae‑; both forms are still in use. So helpio and helpu.