I was wondering if there is a word to describe someone whose name is diametrically oppossed to who they are. For instance a firefighter whose last name is Arson or a swimmer whose last name is Dry.
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(Another word meaning the same thing is "euonym".)
Inaptronym is an ironic form of an aptronym, and examples are given at the link.
Given the linkage in at least one of the references to the word "malapropism", I suggest the neologism malaptronym.
Another possibility (also coined) is misaptronym.
To be more precise, though, the prefix mal- generally means "bad" or "badly" and the prefix mis generally means "wrong" (or "astray"). The prefix anti means "opposite". So a name that means the opposite of what a person does might be antiaptronym. (Contra-aptronym too, maybe, but it's a little hard to say.)
I'd agree with Jim, with the caveat that as aptronym is a Greek-rooted word its opposite should be dysaptronym. But it seems that inaptronym is Wikipedia-canonical.
You would commonly refer to this as a misnomer.
You can also call it a caconym -and therefore caconymous as opposed to euonymous (appropriately named).