The Handshake Was Originally a Peace Gesture

The handshake is believed to have originated in ancient Greece as a gesture of peace — extending the hand showed you were not holding a weapon. Some historians believe the up-and-down motion was designed to dislodge any knives hidden up a sleeve. Today it remains one of the most universal non-verbal signals of trust across dozens of cultures.

Source: Desmond Morris — Gestures: Their Origins and Distribution