1
general::
Phrase(s): ham something up
Fig. to make a performance seem silly by showing off or exaggerating one’s part. (A show-off actor is known as a ham.) • Come on, Bob. Don’t ham it up! • The play was going fine until Bob got out there and hammed up his part.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Exaggerate or overdo, especially with extravagant emotion, as in Hamming up the eulogy was disgraceful, especially since he didn't even know the deceased. It is Also put as ham it up, meaning "overact," as in She loves to ham it up in front of the class. This idiom probably alludes to the hamfat (lard) used to remove stage makeup, mentioned in the minstrel song, "The Ham-Fat Man." From this hamfatter came to mean "an inexpert and flamboyant actor," and was in the late 1800s shortened to ham. The idiom here was first recorded in 1933.
American Heritage Idioms