GOAT as an acronym for "(the) greatest of all time" (also considered as a noun) originally referred to Muhammad Ali, one of the greatest boxers of all time. The first written usage is from 1992 as G.O.A.T. Inc., the name of a company incorporated by Muhammad Ali's wife. The first written usage of GOAT to refer to someone is from 1996. However, GOAT has its roots in mid-1960's when Muhammad Ali was referring to himself as "the greatest of all time". It can be used for anyone and in any field. One can even call their mom "the goat", for example when she cooks their favorite meal. Here is the full origin story from boston.com:
GOAT, as a word used in reference to “Greatest of All Time,” had its origin in an expected place: Muhammad Ali.
In Sept. 1992, Lonnie Ali, Muhammad Ali’s wife, incorporated G.O.A.T. Inc. This company was used as an umbrella for all of the former boxer’s intellectual properties being used for commercial purposes.
According to Sokolowski, the first version Merriam-Webster found of its online use was in 1996 on an Orlando Magic forum referencing Penny Hardaway.
“The sentence is simply, ‘Penny is the GOAT (Greatest of All Time),'” said Sokolowski. “That’s the oldest quotation that we have of this word.”
In 2000, LL Cool J helped to popularize the word as a definition for greatest, when he released an album titled, “G.O.A.T. (Greatest of All Time).” He credited Ali for its origin in a 2016 interview with Rolling Stone.
“Without Muhammad Ali, there would be no ‘Mama Said Knock You Out, and the term G.O.A.T. would have never been coined.”