A person who acts or speaks in a flimsy manner; one who produces weak or insubstantial work (rare or non-standard).
From 'flimsy' plus the suffix '-ist' (one who practices or advocates something). This appears to be a rare or invented formation not found in standard dictionaries.
Words ending in '-ist' usually describe advocates or practitioners of an ideology (like 'socialist'), so 'flimsilyst' would humorously suggest that someone practices 'flimsyness' as a philosophy—it's creative wordplay rather than a real word.
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