The word "Jinkees" originally meant "to tell a lie with the intent of lying about something which is not a fallacy". Obviously, such a confusingly defined word was not used very often when it was first invented, and probably would have been forgotten in favor of other, simpler words like "Yoghurt", "Strignert", "Zibbins", "Inclement" and "Yaoury". Then, Hythmium came along.
It was way back in -2797 EC that the Big Hoax was played. Certain followers of the religion suddenly revealed to the world that the theoalchemical process the religion is based on was non-existant, and insisted that there is, in fact, no way to create metal out of light. When Hythmium himself came forward and demonstrated the process in public, the now ex-followers were discredited. These people, and any after them who claimed that they did not believe in Theoalchemy, are Jinkees.
The word has changed slightly in meaning since then. These days, Jinkees can be anyone who refuses to belive in an obvious truth, although since the definition of "obvious truth" tends to change depending on who you ask, Jinkees can be anyone from the point of view of someone else.
Interestingly enough, the descendants of the original Jinkees formed a club, the Jinkees' Club, dedicated to preserving the "truth" that Hythmium is just a hoax - and, more recently, other "truths" of the same nature. The club still exists today, and there are some surprisingly famous individuals who still claim membership. For instance, Trung Tro Hampher is a long-standing member of the Club, and so was Lady Bavarian Magdelene Creame until her resignation after the imprisonment of her husband, Windsor Creame.