trough (n.)

Old English trog "wooden vessel, tray, hollow vessel, canoe," from Proto-Germanic *trugaz (source also of Old Frisian, Old Saxon, Old Norse trog, Middle Dutch troch, Dutch trog, Old High German troc, German trog), from PIE *dru-ko-, from root *deru- "be firm, solid, steadfast," with specialized senses "wood, tree" and derivatives referring to objects made of wood. Originally pronounced in English with a hard -gh- (as in Scottish loch); pronunciation shifted to "-ff," but spelling remained.