The first thing most people think of when they hear the word yen is the Japanese currency, but yen also means a "desire" or "yearning". The first yen is obviously Japonic, but I thought the second sounded vaguely Gemanic, maybe even being etymologically connected to yearn. Wrong! It's also Asian, but in this case Chinese. In 1900, it was borrowed from the word yen-yen, which meant "a strong craving for opium". This desire got a little less specific over time (possibly with a little influence by yearn), and that's how we were left with the modern word. Yen was the word for "opium", and the repetition implied just how much opium was desired. This might be from jin, meaning "smoke", which has an obscure etymology due to it being a Beijing dialectal word. Although there may be a little interference from the other yen, usage of the word has fluctuated upward throughout the decades.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.