welsh

1. fail to honour (a debt or obligation incurred through a promise or agreement).

   banks began welshing on their agreement not to convert dollar reserves into gold

mid 19th century: of uncertain origin; perhaps from Welsh, on account of the formerly alleged dishonesty of Welsh people.

adjective

relating to Wales, its people, or their language.

1. the Celtic language of Wales, spoken by about 500,000 people (mainly bilingual in English). Descended from the Brythonic language spoken in most of Roman Britain, it has been strongly revived after a long decline.
2. the people of Wales collectively.

Old English Welisc, Wælisc, from a Germanic word meaning ‘foreigner’, from Latin Volcae, the name of a Celtic people.