what is the meaning of this nursery rhyme?
It  referred to resentment at the heavy taxation on wool dating back in 1275.Recently the rhyme has been connected to the slave trade in southern United states
Baa, Baa, Black Sheep
The Real Personages of Mother Goose" (1930) suggested the rhyme referred to resentment at the heavy taxation on wool. This has particularly been taken to refer to the medieval English "Great" or "Old Custom" wool tax of 1275, which survived until the fifteenth century More recently the rhyme has been connected to the slave trade, particularly in the southern United States. 
who wrote the rhyme baa baa black sheep UNANSWERABLE what is it? It is an English nursery rhyme what is its origin? It was published in 1744
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Baa, Baa, Black Sheep [SEP] Origins and meaning
As with many nursery rhymes, attempts have been made to find origins and meanings for the rhyme, most of which have no corroborating evidence. Katherine Elwes Thomas in "The Real Personages of Mother Goose" (1930) suggested the rhyme referred to resentment at the heavy taxation on wool. This has particularly been taken to refer to the medieval English "Great" or "Old Custom" wool tax of 1275, which survived until the fifteenth century More recently the rhyme has been connected to the slave trade, particularly in the southern United States. This explanation was advanced during debates over political correctness and the use and reform of nursery rhymes in the 1980s, but has no supporting historical evidence.