The superlative form of carty; most resembling, containing, or relating to carts, or most prone to traveling by cart.
From 'cart' + the suffix '-y' meaning full of or resembling, made superlative with '-est'.
This is an extremely rare superlative that you'd encounter only in historical or whimsical writing—describing perhaps the 'cartiest' medieval village or the 'cartiest' mode of travel in old farming communities.
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