Definition of "scummy"

Now that the sun was getting high it drew thin sickly looking clouds of poisonous vapour from the surface of the marsh and from the scummy pools of stagnant water.

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887

And what a scummy man / just give him half a chance / I bet he'll rob you if he can.

2006, Arctic Monkeys (lyrics and music), “When the Sun Goes Down”