Possibly because of the not-to-politically-correct phrase "nappy hair" which refers to the tightly curled hair of many people of African ancestry.Why do you think that it relates to hair?
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It is blindingly obvious to me but maybe I look at poetry differently. This is a twist of a very old saying with a new slant.
If you can't stand the heat get out of the kitch(en) and take a nap in the back(room).
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