What explanation do the two men give when Kino asks what they will do if the big wave comes back?

When Kino asks what they will do if the big wave returns, the men explain that there had been a big wave in their great-grandfather's time, but their grandfather had come back, and though there had been a big wave in their father's time, they are now coming back also. "The big wave may never come back" (page 48).