ISIS is typically described as having seized "vast swathes" of Iraq. The word swathe is not normally used in such a context, and its usage here probably reflects the difficulty in conceptualizing the extent of control under the Caliphate. The British pronounce and spell "swath" as "swathe" - the accepted spelling and pronunciation of the word in the UK. The word swathe is from the Middle English, from Old English *swæth; akin to Old English swathian to swathe - a footstep or trace; akin to Middle High German swade swath. The first known use was before the 12th century.
A swath is a long, wide strip of land, or an area of grass or grain that has been cut or mowed. The word may be used in reference to a row of cut grain or grass left by a scythe or mowing machine, or the sweep of a scythe or a machine in mowing or the path cut in one course. It is also seen as a long broad strip or belt; a stroke of or as if of a scythe; or a space devastated as if by a scythe.