[Francis]

The most common reference is noted in the article. As you know during the Ages, Jousting was a "sport". the participants dressed in Armor covering their entire body including their heads. As such, the participants would lift up the "visor" with their hands/finger tips and make eye contact with their opponent. that was a sign of acknowledgemet and respect. Once acknowledged they let go, and the visor would drop to protect their eyes. thereafter the joust begins.

I had heard and was told a military history professor that the reference to the Romans and approaching them showing the hands, is the origin of the "hands up" when you are about to surrender or be searched.

The tipping of the hat is akin to the head nod, acknowledging a passer by.

Now what about the comtempory "fist bump" - no clue