Should a hyphen be used when constructing words using suffixes such as -ly and -wise when the resulting word isn't in the dictionary? For example:
Which one is better?
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Neither is inherently "better". Take, for example, crab-wise, which appears thus in A thesaurus of traditional English metaphors (published only 20 years ago), despite the fact that most dictionaries discarded the hyphen long ago.
As a general principle I think it's better to avoid unnecessary hyphens, so I'd say if you see the unhyphenated version of anything in any credible dictionaries, just copy them.
I shan't feel hard done by if this question is closed as a duplicate of When is it necessary to use a hyphen in writing a compound word?, but I do think the specific issue of "What if it's not in the dictionary?" isn't really covered there.
Even more specifically, "What if the dictionaries disagree?" isn't covered. For example, most dictionaries hyphenate hard done-by, but I'm not going to take issue with Collins in my first link (who didn't). In that particular case one would avoid collapsing it into hard doneby because it would be tricky for the average reader to recognise the component words.
EDIT: I can't believe nobody has yet voted to close this as a dup. But here's another candidate - my own Can word-hyphenation ever be semantically significant?.
What I find fascinating about that one is most votes support answers saying the hyphen can be significant. After thinking about it, I've decided it probably can't, really. If context doesn't already imply the relevant word associations, it's some kind of orthographic game, not real language.