The handshake started as a way to prove you weren't about to kill someone

[Quarry16]

Historians trace the handshake back at least to ancient Greece and Babylonia, and the leading theory about its origin is refreshingly practical rather than ceremonial. In a world where people routinely carried weapons, extending an open, empty right hand toward a stranger was a simple, visible way to demonstrate you weren't concealing a knife or preparing to draw one.

Some historians point to an even more cautious variant, the forearm grip, where two people grasped each other's forearm rather than just the hand, specifically to check for a hidden blade tucked up a sleeve. That extra layer of suspicion eventually fell away as the custom spread and formalized, leaving the simpler handshake we recognize today.

The gesture picked up its now familiar shaking motion much later. Etymological records show the English phrase to shake hands dates to the 16th century, with the specific noun handshake only appearing in print around 1801, suggesting the greeting slowly evolved from a static gesture of open palms into the active, up and down motion associated with sealing a deal or a decision.

Handshake - Etymology, Origin & Meaningwww.etymonline.com