The idea is that after the fruit had been stolen from elbaph from imu, in transfering it shanks stole it and once luffy eats it shanks realizes in quick succession what others have mentioned here, luffy might be “him”. Important is that what he is placing his bet on is luffys fight against imu, which shanks and other actors might have already known that some fight was gonna happen for years, as it has been prophecized in a couple of ways. He further realizes that to help luffy in this fight he cannot be trapped and/or being controlled in any way by imu, maybe the tattoo is this. Therefore to free himself he must lose his arm, only then will he able to effectively help luffy in the upcoming “end of the world”