Lacking fins, or figuratively, lacking elegance, refinement, or the defining characteristics of something fine.
From 'fine' or 'fin' plus '-less' (without). The suffix '-less' creates the opposite meaning, so 'fineless' means devoid of fineness or refined qualities.
The '-less' suffix is one of English's most productive word-builders—you can add it to nearly anything to mean 'without it,' which is why English speakers can understand 'breathless,' 'pointless,' and 'fineless' instantly.
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