What signal would you give to a helicopter pilot who was landing in a bad spot? Such as the top looks like grass, but it's really the top of a six-foot-deep swamp?
The helicopter "wave off" signal is to wave your arms back and forth above your head, first outstretched then crossing above your head, then then uncrossed and outstretched again. Repeat.
Note the possibility for confusion, as many people might do this instinctively when trying to attract attention. However, to a helicopter pilot this means "don't land!"