Past tense of 'ham'; to overact or perform with exaggerated theatrical gestures and emotions.
From 'ham' (an incompetent or exaggerated actor), itself possibly from 'hamfatter' (19th-century theatrical slang), with '-ed' as the past tense marker.
The term 'ham it up' became slang exactly when theater moved from subtle craft to crowd-pleasing spectacle—actors who 'hammed' roles were criticized by serious performers but loved by audiences.
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