"partly decomposed vegetable matter abundant in moist regions of northern Europe," where, especially in Ireland, it was an important source of fuel, c. 1200 in Scottish Latin, of unknown origin, probably from a Celtic root *pett- (source also of Cornish peyth, Welsh peth "quantity, part, thing," Old Irish pet, Breton pez "piece"). The earliest sense is not of the turf but of the cut piece of it, and the Celtic root may be from the same PIE source as piece. Peat-bog is by 1775; peat-moss (mid-13c.) originally was "a peat bog;" the meaning "sphagnum moss" (the type that grows in peat bogs) is by 1880.
