c.1200, in Scottish Latin, of unknown origin, probably from a Celtic root *pett- (cf. 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 connected to that of piece.
Partially decayed vegetable matter, especially peat moss, found in bogs. The low levels of oxygen and the acidic environment in bogs prevent the degradation of peat. Peat is burned as fuel and also used as fertilizer. See more at bog.