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".
(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 would call that an ironic name (warning, tvtropes).
You would commonly refer to this as a misnomer.
a misapplied or inappropriate name or designation.
an incorrect or unsuitable name or term for a person or thing, that conveys a misleading impression.
You can also call it a caconym -and therefore caconymous as opposed to euonymous (appropriately named).
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.
There are two types of these names: ironic ones which are quite coincidental, like Angela Slim, the 500lb bariatric patient or Martin Fish who drowned in Lake Huron. Then there are those which are given monikers, like my chihuaua named Goliath and my bullmastiff we call Ms Tiny. The latter, being deliberate are planned misapronyms and are meant to invoke humor and but the former unfortunate ones are likely 'inaptronyms'. It might be safer to prefix according to the intent.