Green’s Dictionary of Slang

(UK gambling) a form of cheating with dice.

1663		Proposal to Use no Conscience 3: Holding one or two Dice at the top of a Dice-Box, which we Gamesters call Topping [DU].
1669		Nicker Nicked in Harleian Misc. II (1809) 109: [They] wheedle him into play, and win all his money, either by false dice [...] or by palming, topping, knapping, or slurring.
1674		C. Cotton Compleat Gamester 14: Another way the Rook hath to cheat, is [...] by Topping, and that is when they take up both dice and seem to put them in the Box, and shaking the Box you would think them both there, by reason of the ratling occasioned with the screwing of the Box, whereas one of them is at the top of the Box between his two forefingers, or secur’d by thrusting a forefinger into the Box.
1680		Head Eng. Rogue IV 226: You must sometimes use Topping; that is, by pretending to put both Dice into the Box, whereas you have dropt but one, holding the other between your fore-fingers.
1714		T. Lucas Lives of the Gamesters (1930) 136: He sometimes us’d Topping; which is, by pretending to put both dice into the box, but still holding one of ’em betwixt his fingers, which he would turn to his advantage.