The comparative form of 'acid,' meaning more sour or acidic than something else.
From Latin 'acidus' (sour) with the English comparative suffix '-er' added. This is the regular way English forms comparative adjectives from one-syllable words.
When you taste a lemon and then a lime, you might say the lime is 'acider'—though most people say 'more acidic' nowadays, the old comparative form still works and shows how English builds comparisons.
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