above board
adjective phrase & adverb phrase

- legitimate, honest and open

(Oxford Dictionary)

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One of the common theories about the term "above board" is that board refers to the deck of a ship and from the seafaring practice of concealing pirates below decks (below board) in order to make other ships believe there was no danger.

The more likely source is card games, which has enhanced the English language with a large variety of expressions. In this case, above board probably refers to the idea of keeping one's hands above the table or board so as to prove that one is playing fairly (or at least give the appearance of being fair).

The opposite, placing one's hand under the table and perhaps not playing fairly, produced the expression "underhanded," as well as "under the table" and "under the counter," both of which refer to dishonest or illegal dealings.

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SYNONYMS

bona fide, ethical, fair, honourable, on the up and up, reputable, trustworthy, upfront, upright, virtuous

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SMUGGLE OWAD into today's conversation

"I can assure you that everything in our contract is ABOVE BOARD."