Green’s Dictionary of Slang

knock-in n.

[ety. unknown; ? one ‘knocks in’ or plays one’s cards]

the game of loo; thus a hand at any card-game.

1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.