The side opposite left. Also what one is owed — the same word for a direction and for a just claim.
Old English riht (straight, correct, just), from Proto-Germanic *rehtaz, from a Proto-Indo-European root for moving in a straight line; one old word for both a direction and a just claim, since both trace to what does not bend.
A single word doing two jobs at once — the side that is not left, and the claim that is owed. Both senses descend from the same old idea of what runs straight and true.