dioptóngón
Definitions (by source)
‘diphthong’ 8.15; nom. pl. diptongoin 9. Also without -ón ending in dat. pl. diptangaoighibh 8.13, diptongaidhibh 10.1
Aibidil Gaoidheilge & Caiticiosma: Seaán Ó Cearnaigh's Irish primer of religion, published in 1571. Editor: Brian Ó Cuív.