Green’s Dictionary of Slang

open, honest; thus extrapolation over the board in cit. 1996.

1620		R. Carpenter Conscionable Christian (1623) 118: All his dealings are square and above the boord [OED].
1686		W. De Britaine Hum. Prud. xvi 74: Keep formality above board, but Prudence and Wisdom under Deck [OED].
1757		Scots Mag. 5 Dec. 6/2: I had done nothing but what was open, above baord, and with permission.
1836		Carlisle Patriot 19 Nov. 3/1: The accounts were all right, straight, and above board.
1842		Cumberland Pacquet 20 Sept. 3/3: The wrestling throughout was good, and [...] all was fair and above board.
1859		H. Kingsley Recollections of G. Hamlyn (1891) 121: That shall be the game for me in future, Polly; all square and above-board there.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 154/2: There was cheatin’ a goin’ on; it wasn’t all fair and above board.
1868		Bell’s Wkly Messenger 14 Dec. 4/4: Here everything is fair and above board.
1884		Peterhead Sentinel 6 Feb. 1/2: Genuine Sale all the year round [...] no clap-trap whatever; all fair and above board; but we cannot stand priggin’ — na hae seen.
1891		Weekly Reporter 239: He may do the work [...] provided everything is fair and above board.
1908		E.S. Sorenson Quinton’s Rouseabout and other Stories 115: I tell yer, I begun ter feel real queer. Things didn’t seem to be above board somehow.
1910		Wodehouse Gentleman of Leisure (2003) 20: Thash — thash not point. It’s principle of thing. Have thish thing square and ’bove board, I say.
1916		‘A-No. 1’ Snare of the Road 20: My rising suspicion that all was not above board with the intentions of Carolina Bob.
1920		East Oregonian (Pendleton, OR) 7 May 1/2: To protect themselves against unfair competitioon [...] ‘above board’ publishers join the Audit Bureau of Circulation.
1923		Wodehouse Leave it to Psmith (1993) 397: Let all be open and above-board.
1927		W. Edge Main Stem 95: You have a sort of open and above-board way of doing things that attracts attention.
1934		Wodehouse Right Ho, Jeeves 185: How are we to know that this has all been open and above board?
1940		M. de la Roche Whiteoak Heritage (1949) 99: Just tell your family, since you’re so above-board, what time you came home last night.
1959		D. Niland Big Smoke 176: It’s all so simple and above board.
1969		R. Rendell Best Man To Die (1981) 25: Generous to a fault, and if he wasn’t strictly above-board – well, a man had to live.
1976		C. Dexter Last Seen Wearing in Second Inspector Morse Omnibus (1994) 418: I don’t think he’s completely above-board with me.
1989		T. Blacker Fixx 85: Just a nice little business. Making and selling. All above board.
1996		J. Cameron It Was An Accident 217: Them things weren’t all exactly totally over the board.