2、depth:  Depth is not as old as it looks. Similar nouns, such as length and strength, existed in Old English, but depth, like breadth, is a much later creation. In Old English the nouns denoting ‘quality of being deep’ were dīepe and dēopnes ‘deepness’.=> deep
4、late 14c., apparently formed in Middle English on model of length, breadth; from Old English deop "deep" (see deep) + -th (2). Replaced older deopnes "deepness." Though the English word is relatively recent, the formation is in Proto-Germanic, *deupitho-, and corresponds to Old Saxon diupitha, Dutch diepte, Old Norse dypð, Gothic diupiþa.