(Said with astute Cliff Claven voice from 'Cheers' fame) I recall reading long ago that the custom of shaking the hand of another in greeting came from ancient customs of confronting a potential adversary whom may be armed. The open hand prior to the clasp was meant to demonstrate that there was no weapon in hand.
Guess this was prior to concealed carry!
Anyhow, I have never researched this bought since it came up, I thought that it was interesting.