Def: the original or mother wave (old Norse); used, as legend would have it, by experienced Shetland fishermen prior to the days of compasses to find land in times of fog. Described as a 'surge or physical protest' the ocean makes when her cosmic motion is restricted by proximity to land. A 'sensed' undercurrent, which is a wave-like motion or swelling running towards land, discernible to the trained eye on soundings and observable in foggy weather .
'No matter how wind-driven and uncertain the billow, the methodical undulations of the 'moder-dye' can be seen across the hills and valleys of a wind tormented sea, always setting four-square towards the land'. (Sail Fishermen of Shetland, p74)