Oggham

The age-old runic language of the Oggs, referenced in Lords and Ladies and A Hat Full of Sky. It was originally a "very ancient" Dwarven language, but has also been used by the old mound-builders of Lancre, at least during the building of the The Long Man.

The language must still be known by dwarves to some degree, since Zakzak Stronginthearm uses it as decoration, but – perhaps by its connection with the Oggs – the two documented occurrences on the Discworld are rather rude in nature.

On Roundworld

Ogham runes, or the tree-alphabet, were beginning to emerge as a written Celtic script at the time the Celtic world was largely over-run by the Romans - they persisted for a long time in Ireland, (influenced by Nordic runes) although Britain and Gaul adopted the Latin alphabet.

Revived in our day as a part of modern Paganism and witchcraft, (of the Magrat Garlick folklorique sort).