Capable of being amazed or shocked by something surprising or unexpected.
From astound (from Old French 'astoner', meaning to stun) plus the suffix -able (meaning capable of), creating an adjective describing something that can be astounded.
This word is rarely used in modern English, but it reveals how our language lets us add -able to almost any verb to create new adjectives—'astoundable' technically exists even if most people wouldn't say it!
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