Most cap-like or most resembling a cap; superlative form of cappy.
From cap plus -y plus -est (superlative suffix), creating the superlative adjective meaning 'most cap-like,' following English morphology rules.
The superlative 'cappiest' is so obscure that it's practically a ghost word—most English speakers would never use it, but it technically exists as the grammatically correct superlative of 'cappy!'
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