trough (n.) Look up trough at Dictionary.com
Old English trog "wooden vessel, tray, hollow vessel, canoe," from Proto-Germanic *trugaz (cognates: Old Frisian, Old Saxon, Old Norse trog, Middle Dutch troch, Dutch trog, Old High German troc, German trog), from PIE *dru-ko-, from root *dru-, *deru- "wood, tree" (see tree (n.)). Originally pronounced in English with a hard -gh- (as in Scottish loch); pronunciation shifted to "-ff," but spelling remained.