Restive means "incapable of staying still". Well, then why does it have the root rest in it? Isn't that its own opposite? Indeed. Restive used to be its own antonym as little as 500 years ago, and to understand why it changed we need to understand horse domestication. A horse that didn't want to move was called restive, meaning that it stayed still. However, these horses stayed still because they were nervous, and that also was extended to make them jumpy and nervous. From there, you can easily see how the modern meaning evolved, as the other one lost prevalence. Anyway, in Middle English, restive was restyffe, from Middle French restif, still "motionless", from Old French rester, "to remain", which, yes, is from the root rest. This may be from Proto-Germanic, but it was more likely Latin; if the latter is the case, then it is an affixation of re- ("backward", from PIE wre, "again") onto stare ("stare", from PIE steh, also "stare").
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.