Green’s Dictionary of Slang

a bribe, a tip, a present; also as v. (see 1967 cit.).

1818		J. McLeod Voyage of His Majesty’s Ship Alcestis in DSUE (1984).
1832		R.M. Martin Past and Present State of Tea Trade 164: I take the register of the ships engaged in the China trade in 1786 and those in 1830-31, with the amount paid in present or cumshaw.
1838		C.T. Downing Stranger in China I 193: Generally speaking, the value of the cumshaw is about five per cent, on the bill paid; but sometimes, [...] eight and even ten per cent, is allowed.
1851		J. Blunt Shipmaster's Assistant 590: The ship’s comprador will expect a cumshaw or present of $200 or $300, according to the number of the crew ; and if you employ the hoppo man, or custom-house officer, in his stead, the linguist will expect a cumshaw of $200.
1864, 1867, 1870		Hotten Sl. Dict. 113: CUMSHAW, a present or bribe. — Anglo-Chinese.
1873		Sl. Dict.
1873		Overland Mthly Oct. 323/2: A hundred hands were stretched out for a present. " Cumshaw, cumshaw, cumshaw!" It sounded right and left, before and behind.
1885		Where Chineses Drive 163: Baldpate [...] had the exceeding coolness to ask for a cumshaw as they left .
1887		G. Davis Recoll. Sea-Wanderer 160: She will also fill the mattresses with fresh bamboo shavings [and] when the ship leaves for home, giving to each one a 'cumshaw' or present, generally a large pot of preserved ginger.
1899		C.J.C. Hyne Further Adventures of Captain Kettle 119: The Coast nigger hereabouts has got a fancy that any ship on the beach is cumshaw for himself.
1904		Truth (London) 16 June 1514/2: I infer that ‘cumshaw’ is the Service slang for what has previously been spoken of in truth as ‘backsheesh’.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 69: Cumshaw: Naval slang for something ‘extra’; something gratis; a perquisite; a gift, equivalent to Buckshee.
1935		A.J. Pollock Und. Speaks 28/1: Cumshaw, the pay off; bribe money; shakedown.
1941		C.R. Bond 20 Jan. in A Flying Tiger’s Diary (1984) 77: We got in a good bull session about the graft in China [...] Everyone seems to get his cut – ‘cumshaw’.
1958		(con. 1950) E. Frankel Band of Brothers 34: What kind of men were these to give away things of such value, and then deprive a man of cumshaw?
1967		(con. 1940s) M. Dibner Admiral (1968) 166: I’ve cumshawed every fighting ship I could from the Atlantic Command and they’re squawking like hell.