In Welsh and Cornish the usual word for good is da [daː], while in the other Celtic languages words for good are: Breton – mat [maːt˺], Irish – maith [mˠa(ɪ)(h)], Manx – mie [maɪ], and Scottish Gaelic – math [ma]. I’ve wondered for a while whether there were cognates in Welsh and Cornish for these words.
Last week I found that there are: mad in Welsh and mas in Cornish. The Welsh word, which means good, seemly, lucky, appears in the phrase: a wnêl mad, mad a ddyly (one good turn deserves another), but isn’t otherwise used, as far as I can discover. The Cornish word doesn’t appear in the Cornish dictionaries I’ve checked so I think it is probably not used any more.
These words all come from the Proto-Celtic *matis (measure), possibly from the Indo-European mē (measure, consider) [source], which is also the root of the Irish word meas (judgement, opinion, respect) [source], and possibly of the Welsh meddwl (to think), and the English mete (measure).