https://en.m.wikipedia.org/wiki/Etymology_of_ham_radio

At the turn of the 20th century, the terms “ham” and “plug” were used by landline telegraphers to describe an operator “who lacks ability”[2] or who had poor or “ham fisted” skills.

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Even among amateur radio operators, the term was used pejoratively at first by serious experimenters. For example…

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But many other amateurs increasingly adopted the word “ham” to describe their hobby and themselves during this period, embracing the word that was originally an insult…

(Just some quick highlights fromn the wiki. I recommend reading the whole thing.)

HAM stands for amateur radio operator. That’s originally pejorative usage (like “ham actor”) for amateur radio operator by operators in commercial and professional radio communities.

It’s not an acronym. It isn’t traditionally capitalized, just a word: ham. Like the stuff you put in sandwiches.

My earliest understanding of it is that it started out when ham radio was brand new, and there was some rivalry between commercial radio and hobbyist radio. Commercial radio people started calling hobbyists “hams” with the same sentiment that applied to actors that overacted; they were aloso hams, of course. As to where that came from, I couldn’t say.