Comparative form of fusty; more musty, moldy, or smelling stale; more old-fashioned or stuffy in outlook.
From fusty, the comparative form showing progression of the quality of staleness or mustiness. Fusty itself derives from fust, an archaic noun meaning mustiness or a stale smell.
The fact that we have comparative forms like 'fustier' and superlative 'fustiest' for staleness shows how much it mattered to people in older times—smell and decay were such visceral concerns that describing gradations of mustiness was essential vocabulary.
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