In case anyone is wondering, the word "ham" in this sense derives from "ham fat" (ветчинный жир), which was formerly used in amateur theaters as an inexpensive base for making грим ("stage makeup").

Thus, "a hamfatter" was originally an inexperienced amateur actor, and later, a professional actor whose performance is artificial and/or lacks subtlety and/or is full of cliches. So, in the context of film/theater, "to ham it up" is a synonym for:

  • "to overact" or
  • "to be excessively theatrical" or
  • "to be too melodramatic"


And the original slang "a hamfatter" is now very rare and old-fashioned -- we simply say "a ham."

Real-life example: Al Pacino is often named as an actor who was very good in his early films, but became increasingly "hammy" in his later years -- he stopped playing characters, and instead we got Al Pacino playing the role of Al Pacino. In fact, some comedians say that it's pointless to do an "Al Pacino impression," because Al Pacino has been doing a parody of himself for so many years.