I'm guessing, but one old slang meaning for 'meg' was a guinea...ie �1.05...and similarly for 'keg' was a promiscuous woman/tart. It's possible that a 'meg keg' might just mean "a cheap tart". Is the phrase used only for women?
Yes I haven't heard it said about a man anyway, its usually suggesting dressed in their finery, it makes sense about the guinea, not sure about the tart!!!