Pad thai is ubiquitous street food in Thailand. Its original name, kway teow pad thai, combines the Chinese term for rice noodles (kway teow), the Thai word for stir-fry (pad), and of course, "thai," meaning Thai-style. As the name and the noodles suggest, the roots of this dish likely date as far back as the 1700s, when the port city of Ayutthaya drew merchants and settlers from southern China. The official origin of modern pad thai (and it is more official than most!) was the result of a coordinated government effort during World War II. If you'd like to learn more, here's an article with lots more detail.