Decomposition Row, n. — Green’s Dictionary of Slang

Decomposition Row n.

[a pun on Rotten Row, itself ult. route du roi, the royal road, from Kensington Palace and St James’s]

Rotten Row, the track in Hyde Park frequented by fashionable horseriders.

1859		London Dly News 7 Sept. 2/5: The little wax hands of the belle of the season [...] riding through Decomposition Row towards Kensington .
1862		Leeds Times 19 Apr. 6/1: Pretty young maidens, and aristocratic dashing young bachelors [...] the frequent ornaments of ‘Decomposition Row’.
1864, 1867, 1870		Hotten Sl. Dict.