ham

Inherited from Middle English hamme, from Old English hamm (“inner or hind part of the knee, ham”), from Proto-West Germanic *hammu, from Proto-Germanic *hamō, *hammō, *hanmō, from Proto-Indo-European *kónh₂m (“leg”).

Cognate with Dutch ham (“ham”), dialectal German Hamme (“hind part of the knee, ham”), dialectal Swedish ham (“the hind part of the knee”), Icelandic höm (“the ham or haunch of a horse”), Old Irish cnáim (“bone”), Ancient Greek κνήμη (knḗmē, “shinbone”). Compare gammon and gam.

It came naturally from the word amateur. Deemed likely by Hendrickson (1997), but then the question would be why it took so long to pop up. He rejects the folk etymology of Cockney slang hamateur because it originated in American English.

From the play Hamlet, where the title character was often played poorly and/or in an exaggerated manner. Also deemed likely by Hendrickson, though he raises the issue that the term would have likely been around earlier if this were the case.

From the minstrel's practice of using ham fat to remove heavy black makeup used during performances.

Shortened from hamfatter (“inferior actor”), said to derive from the 1863 minstrel show song The Ham-fat Man. William and Mary Morris (1988) argue that it's not known whether the song inspired the term or the term inspired the song, but that they believe the latter is the case.