'above-board' originates from English, formed from 'above' + 'board' where 'board' meant 'table' or 'plank' (i.e., the gaming or trading table); literally meaning 'on or above the board'.

'above-board' emerged in the mid-18th century (used in contexts such as card-playing and trade where keeping one's hands above the table showed nothing was hidden) and developed into the idiomatic phrase 'above board' meaning open and honest.

Initially it described a literal position ('hands above the board/table' to show nothing was concealed); over time it evolved into the general sense 'open, honest, and legitimate.'