Resembling, containing, or having the properties of alum; slightly tart or astringent in taste.
From alum + -ish (somewhat like). A descriptive adjective from Middle English following the pattern of -ish adjectives describing taste and texture.
Alum tastes sharp and puckering—like biting into an incredibly sour grape—so 'alumish' describes that characteristic astringent sensation your mouth gets from alum salts.
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