T. S. Eliot's poem describes Macavity as a ginger cat. What does it mean?

The term "ginger" is used in British English to refer to a yellow-orange color resembling that of the dried spice ginger, made from the ground root of the ginger plant and commonly used in many Asian cuisines. It applies to cats and also to human redheads. 

The term "ginger" refers in this context to a pale orange colored cat. The term "marmalade" is sometimes also used in British English to refer to an orange tabby cat, while "ginger" is an orange colored cat with or without distinctive tabby markings. A picture of a typical "ginger' cat can be seen at: Warren Photographic.

Eliot's description of Macavity suggests a tall, rangy, feral alley cat rather than a plump, well-fed house cat. The criminal nature of the moggie (British slang for cat with no particular pedigree) suggests a free-ranging cat who has adapted with cunning and athleticism to the urban environment.