This article discusses the contention that the oath modebroth, attributed to St Patrick in a number of Irish sources, is a Brittonic phrase, as stated in Sanas Cormaic. This has been accepted by several modern linguists. Here, an alternative native Irish etymology is proposed for the oath. Neither explanation can be disproved on linguistic grounds. Examination of the treatment of modebroth by other glossarians shows that Sanas Cormaic is the only extant source to identify it as Brittonic, and the hypothesis is advanced that this may have been a deduction on the part of the author rather than an inherited tradition. These considerations cast doubt on the reliability of regarding modebroth as Brittonic.