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A barking dog seldom bites.
Literal
Bark dog [topic-は] seldom not-bite [nature-ものだ].
A proverbial statement. 吠える犬 is a relative clause modifying 犬 — '[the dog that] barks.' めったに ('rarely, seldom') always pairs with a negative. ものだ at the end marks this as a general truth or proverbial wisdom — 'this is how it is, as a rule.' Classic fixed saying.