Rearmost means furthest back. The rearmost seat is the one at the very back of the bus. The rearmost car of a train is the last one. Rear is the back; -most means “the furthest in that direction,” the same ending you meet in foremost (furthest forward). Read it as rear + most, not as re-arm. It has nothing to do with weapons or with the verb rearm, even though the letters look close enough to trick a tired eye. It is a neat, slightly bookish word; many people just say “the last” or “the one at the back.” Though actually, rearmost is useful when you need the exact back-most one in a line, not merely one that is quite far back.