That ain't no 'lady', by the sound of things!
As with all vulgar phrases and expletives, there is no real meaning behind the words. What matters is the 'force' of the words, which showing anger, surprise, disgust, or any other emotion.
This phrase is usually used to mean 'I don't care'. It's a power thing. Someone might use this as a response to show that they are strong and resilient, and not worried or upset by others. Somebody insults you, or tells you to do something you're not willing to do, for example, and you respond by (figuratively!) not only showing them your behind, but telling them to kiss it.