Old Frisian ‘thorp’, ‘therp’ or ‘wir’. See: Altfriesisches Wörterbuch, pp. 1076. In modern Fries and the Groninger dialect, the words ‘terp’ and ‘wierd’ mean: an inhabited artificial hill or mound. See: Historical Dictionaries.
Many terpen are located in the northern Netherlands and existed as early as the Middle Ages. Many still exist to this day, with one building or sometimes holding a whole village. More about the remarkable history of these mounds can be found on this web site (also available in English): Terpenwiederenland.