Comparative form of bumpy—more rough, uneven, or jolting in a way that causes movement up and down.
From 'bumpy' (adjective from 'bump') plus '-er' comparative suffix (standard English grammar for adjectives of 2+ syllables).
We unconsciously describe road conditions with '-y' adjectives (bumpy, lumpy, lumpy, potholed)—our language mirrors the physical sensation of bad pavement.
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