Green’s Dictionary of Slang

a present of money; also an extra portion added for free to a purchase.

1848		J.H. Stocqueler Oriental Interpreter 58/2: CHERRY-MERRY, the vulgar phrase for Buxis, or Bukshish (presents), chiefly, if not solely, in use in Western India.
1855		M. Rafter Percy Blake 249: As I [...] never abused or ill-treated them, and always gave them cherry-merry when they attended my hunting-parties, they looked upon me as one of a superior species to the ordinary run of my countrymen.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1881		Morn. Bull. (Rockhampton, Qld) 2/7: The words, ‘tip,’‘palm oil,’‘cherry merry,’ and ‘douceur,’ are synonymous.
1910		Dly News (Perth) 18 July 9/4: ‘Salaams’ in their most degenerate form have a moneys value too, as when the urchin in the street says – ‘Salaam’ Sab! and forthwith cries – ‘Cherry Merry dio,’ Sab! One may despise the urchin for this.
1911		Methodist (Sydney) 16 Dec. 9/2: In India, whenever any man comes to sell anything, no matter what it is, I always noticed not only would he fill his vessel to overflowing, but gave an additional quantity which they call cherry, merry.