There's a feature on the Welsh language on the BBC website (here). From it I learned that some people consider the word balderdash to have come from the Welsh baldordd, which means 'idle, noisy talk or chatter'. The OED mentions this possibility, but also raises the possibility that balderdash may have a Germanic root.
Other English words from Welsh include corgi (with the literal meaning in Welsh of 'dwarf dog'), flummery (a dessert similar to blancmange), which is an anglicised spelling of the Welsh llymru, the ll sound being notoriously difficult to pronounce or to express using the English alphabet (the surname Floyd, similarly, comes from the Welsh Lloyd), and gull (the OED says that it perhaps comes from the Welsh gŵylan). Truant is also mentioned in the BBC piece; the OED says that this entered English from Old French, although concedes that it probably derives from a Celtic source, and does make a comparison with the Welsh truan, wretched or a wretch.