in the open; without dishonesty, concealment or fraud
The adverb above board originally meant with one’s cards visible above the level of the board (that is, the playing table), so as to avoid suspicion of cheating. In A Dictionary of the English Language (1755), Samuel Johnson wrote:
Above-board. In open sight; without artifice or trick. A figurative expression, borrowed from gamesters, who, when they put their hands under the table, are changing their cards. It is used only in familiar language.
This adverb is first recorded, and with reference to card playing, in The terrors of the night or, A discourse of apparitions (1594), by the English pamphleteer Thomas Nashe (1567-circa 1601):
Senior Sathan when he was a yong stripling, and had not yet gotten perfect audacitie to set vpon vs in the day time, was a sly Polititian in dreames; but those dayes are gone with him, and now that he is thoroughly steeled in his scutcherie, hee playes aboue-boord boldly, & sweeps more stakes than euer he did before.
The first known figurative use of above board is found in Lenten Stuffe (1599), by the same author:
Two faithful Lovers they were, as every Apprentice in Paul’s Churchyard will tell you for your Love, and sell you for your Money: The one dwelt at Abydos in Asia, which was Leander; the other, which was Hero, his Mistress, or Delia, at Sestos in Europe, and she was a pretty Pinkany [= sweetheart] and Venus’s Priest; and, but an Arm of the Sea divided them: It divided them, and it divided them not, for over that Arm of the Sea could be made a long Arm. In their Parents the most Division rested; and their Towns, that, like Yarmouth and Leostoff, were still at Wrig Wrag [= at enmity], and sucked from their Mother’s Teats serpentine Hatred one against each other; which drove Leander, when he durst not deal above-board, or be seen a-board any Ship, to sail to his Lady dear, to play the Didopper [= small diving water-fowl] and ducking Water-spaniel to swim to her, nor that in the Day, but by Owl-light.