you're very helpful, Michael.

Yes, metaphorically. Native Americans (who used to be called "Indians") were driven off the land they lived from by early (mostly European) settlers in America. Later, as some kind of compensation, the remaining tribesmen were offered (and accepted) land where they could live their lives in their traditional ways. Each such parcel of land was known as a "reservation." So, wandering off the reservation meant leaving your allowed, allocated living space to try something different out in the rest of what became the USA ... So, here, she is "the reservation" - his special territory - and he has wandered off it by going to another girl. https://en.wikipedia.org/wiki/Indian_reservation https://www.urbandictionary.com/define.php?term=off%20the%20reservation https://en.wiktionary.org/wiki/go_off_the_reservation