It's your age...........
Sure it's not a lefthand thread?
Big socket, long bar, longer lump of steel tube over that, if the socket tries to slip off the nut try putting an axle stand under it!
If it's like the ones on a disco 1, drill a hole through the side of the nut (paralell with thread) and split the nut with a small chisel (need a new nut then though), good for a last option.