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It's a reference to the amount of wrinkles on this hypothetical her's face. I actually learned it from the cement rendering crew I worked with. If a cement wall has lots of indents and grooves it will tend to hold on to more water. So we're actually comparing her face to a cement wall.
All the best.

lol! man that's a great explanation @boldgrgunok! a little mean but not unexpected at all, thank you sir!