The origins of are quite difficult to trace actually and I am not sure I have the answer but here goes.
Here is the possible beginnings of the use of the word “copycat”
” A pox upon him for me, he is more and more a cat”. It comes from Shakespeare’s play, All’s Well That Ends Well, written between 1604 and 1605.
What does the person saying this mean in the play? It either means that the person saying the phrase is insulting a person as he does not like cats or he is describing the person as cat-like in that he is able to land on his feet whatever is thrown at him. The modern description is “teflon man” or some such phrase.
We know that cats have an excellent self-righting method that is utilised when he or she falls (see cat falling).
Both of these meanings have no direct connection to the modern meaning which is that a person or persons copy someone’s actions. A typical example would be “copycat killing” in which a criminal kills someone in the same manner as another person.
However, if a person is like a cat they could be said to be copying a cat’s actions. This may have then evolved into the phrase “copycat” over the ensuing hundreds of years.
It was used two generations before 1896 as it was referred to in The Country of Pointed Firs by Sarah Orne Jewett of 1896:
“”I ain’t heard of a copy-cat this great many years..’twas a favorite term o’ my grandmother’s”