Green’s Dictionary of Slang

a knowledgeable, artful, aware person.

1821		Egan Life in London (1869) 321: Mr Mace had long been christened by the downies, ‘the dashing covey’.
1829		‘On the Prigging Lay’ (trans. of ‘Un jour à la Croix Rouge’) in Vidocq IV 262: Lushing blue ruin and heavy wet / Till the darkey, when the downy set. / All toddled, and began the hunt / For readers, tatlers, fogies, or blunt.
1859		Matsell Vocabulum 27: downey A smooth, pleasant talker; a knowing fellow.
1880		W. Hooe Sharping London 35: Downy, a knowing fellow.