"Unrelentless" to mean "relentless"?

Question

A native speaker commended someone for investigating something thoroughly, so they said "for your relentless investigating efforts", then they corrected themselves and said "unrelentless", then finally they thought maybe "relentless" was right after all. I suggested "unrelenting".

Wiktionary states this use of "unrelentless" might be a malapropism. The non-standard variant appears in Ngrams, there has been continued interest for the word (trends), the possible solecism is discussed in the context of other words beyond just discarding it, some opinions from other forums date to almost 20 years back so there is value in seeing if these have changed, and it also appears in some song lyrics (like Alanis Morissette's Big Bad Love (1992) "I wonder why I am so unrelentless / There's nothing that I can do to prevent this...") which seemingly haven't been corrected in the last 30 years.

Do you accept, or is "unrelentless" generally accepted to mean, "relentless" in this context; is it a malapropism or not, a solecism or a non-standard variant, and does that vary with regions? Which of the three adjectives do you find better suited in this context, and why?

Answer 1

Do you accept, or is "unrelentless" generally accepted to mean, "relentless" in this context?

I very much doubt it. As the comments sum up, it's basically confusing unrelenting with relentless, formed on the same parallel as irregardless. (irrespective + regardless)

The same connection was brought up here on Word Reference. But it is not standard English.

I've never heard this mistake before, and I'm pretty sure your friend was just overthinking it in the moment.

Sadly, it's been used in published, academic content:

The decision to pursue MAID could provide considerable relief from the otherwise unrelentless work of illness and dying. [link]

Perfectionism is the unrelentless striving to avoid the guilt or shame of not working towards the inevitable and infinite next step of a rigid policy designed to avoid error. [link]

I'd call it a non-standard variant, one that's been examined as far back as 1925. Garner, in his (well, obviously 'Garner's') Garner's Modern American Usage, has an entry for unrelentlessly, which he describes as a 'nonword':

*unrelentlessly is a solecism for either unrelentingly or relentlessly. Ironically, this NONWORD literally suggests just the opposite of the intended meaning- e.g.:

"He has unrelentlessly [read relentlessly or, better, faithfully] served as a committee person involved in parks and recreation, fire prevention, police and emergency services, highway management, budget control and youth and school advisory committees." Letter of Larry E. Rice, "Change Dewitt Leader,"
[Post-Standard (Syracuse), 19 Oct. 1995, at 15.]

"This prime minister [Benjamin Netanyahu] is still under the shadow of public opinion, and the opposition will unrelentlessly [read unrelentingly or relentlessly] continue to fight against him through legal, moral and political means."
[Amos Perlmutter, "Dodging a Misguided Political Fusillade," Wash. Times, 22 Apr. 1997, at A14.]

Answer 2

"Unrelenting" would definitely have been better.

extremely determined; never becoming weaker or admitting defeat:
She will be remembered as an unrelenting opponent of racial discrimination.

If you describe someone's behaviour as unrelenting, you mean that they are continuing to do something in a very determined way, often without caring whether they hurt or embarrass other people.

Cambridge

Answer 3

Un- means not. -less also means not. So unrelenting and relentless mean the same thing, not relenting.

Unrelentless is not a word, at least not according to my spelling checker :-) If it was a word, it would mean “not not relenting” which seems to be the opposite of what you wanted.