Tongu do dia toinges mo thúath

Tongu do dia toinges mo thúath is an Old Irish oath which translates to 'I swear by the god by whom my people (túath) swear'. It is the standard oath in early Irish literature. Such formulae are common in early Irish literature, and especially in the heroic sagas, where they are sworn for emphasis when a character declares they will perform some feat.

Some scholars have interpreted this oath as a relic of Irish Celtic paganism, preserved in Irish literature. Along these lines, Joseph Vendryes argued that the god behind this oath was the Celtic god Teutates, and Calvert Watkins that the oath had roots in Proto-Indo-European. On the other hand, Ruairí Ó hUiginn has argued that the oath was a scholarly Christian invention, contrived to suit the pagan background of the Old Irish sagas. The aspect of taboo in this oath has also been discussed, with John T. Koch suggesting that it originated as a taboo deformation (i.e., modification of a phrase to avoid a taboo word).