According to the Washington State Bar blog, NW Sidebar, the custom of raising our right hand to be sworn in to court as a witness or juror has rather grim origins. In the way olden days of 17th century London, before written recordkeeping became widespread, some judges punished convicted criminals by branding their right hands. T for Theft, F for Felon, and so on. A second offense cost the convict leniency — hence, raising the hand to show one’s status.
Of such things are long traditions begun.
More details, including citations for you historical fiction writers, on the WSBA blog.
Gives me the willies, it does.