from Wiktionary:
Portuguese têmpora (“Ember days”), from Latin tempora, plural of tempus (“time; period”). When Portuguese explorers (mostly Jesuit missionaries) arrived in Japan, they abstained from eating beef, pork, and poultry during the Ember days, a Catholic series of holidays. Instead, they ate fried vegetables and fish. This was the first contact of the Japanese with fried food, and since then they began associating the Portuguese word têmpora (which they pronounced tenpura) with such food.
