means A long, open container for animals to eat or drink from, or by extension the low point in any wave, cycle, or curve.

from From Old English 'trog,' meaning a hollowed-out vessel or wooden vessel, with relatives across the Germanic languages — German 'Trog,' Old Norse 'trog' — and a deeper cousin in words meaning 'tree' or 'wood,' since the first troughs were simply logs hollowed out. The sense of a 'low point between two crests' is a much later figurative borrowing from the shape of a wave's dip.