''Above Board'' is a Nautical term which originated from The ''Age of Sail'' (17th to 18th Century AD). As Sailing Ships were of wooden construction then, Decks were joined planks or boards. If something was considered ''Above The Board'', it was seen to be in plain sight and not in hiding. Such a term could be applied to the ''turning out'' a Ships Company (Crew) – in it's entirety, on The Main Deck. This could be for Ceremony, Captain's Call (orders) or a variety of reasons. It also showed trust in that no other persons were ''hidden''.
As in Piracy, ''Above Board'' was linked to the practice of hiding the majority of a Crew from a possible ''Prey Ship''. This was important with regards to Boarding Parties. In other applications, ''Above Board'' is related to a Person who is ''straight forward and forthright''.