Disputed; there are two Old Irish nouns son, one meaning ‘sound; word, name’ (a borrowing from Latin sonus), and one meaning ‘prosperity, well-being’ (cognate with sona (“happy”) and sonus (“good fortune”)). The connection with sonus was rejected by the Lexique étymologique de l'irlandais ancien, its authors unconvinced on semantic grounds. The Dictionary of the Irish Language associates the preposition with the first of these, suggesting the original meaning ‘at the sound of, by the word/name of’. Dinneen’s and Ó Dónaill’s dictionaries, on the other hand, associate it with the second, suggesting the original meaning ‘for the well-being of’. Randall Gordon, in his study of Old Irish verbal nouns, associates the phrase with neither word and proposes his own etymology connecting it with Proto-Indo-European *senh₂- (“to reach, attain”), from which imresan (“contention”) is also derived. Cognate with Scottish Gaelic airson in any case.
ar son (plus genitive or possessive determiner, triggers no mutation)
| Person | Normal | Emphatic |
|---|---|---|
| 1st person sing. | ar mo shon | ar mo shonsa |
| 2d person sing. | ar do shon | ar do shonsa |
| 3d sing. masc. | ar a shon | ar a shonsan |
| 3d sing. fem. | ar a son | ar a sonsa |
| 1st person pl. | ar ár son | ar ár son-na |
| 2d person pl. | ar bhur son | ar bhur sonsa |
| 3d person pl. | ar a son | ar a sonsan |