Onn is the Irish name of the seventeenth letter of the Ogham alphabet, ᚑ, meaning "ash-tree".
In Old Irish, the letter name was Onn, which is related to Welsh onn(en), from the root was *ōs-, *osen 'ash'. Its phonetic value is [o].
Onn "ash" was replaced by uinnius. McManus takes this as an indication that the Ogham letter names date to the Primitive Irish period.
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