(or Hamming it or Hamfatter)
These terms imply overacting as a product of egotism or lack of talent. Sometimes ham acting is intentional, in farcical or melodramatic plays intended for broad overplaying, but usually it is undesirable. The term seems to have been born as a result of the habit of blackface entertainers smearing their faces with ham fat before blacking up when playing in "Tom shows," the innumerable productions of Uncle Tom's Cabin that toured the United States during the late 19th and early 20th centuries.
The Historical Dictionary of the American Theater. James Fisher.