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The longest word in the Old Testament is Mahershalalhashbaz, 18 characters. The name is found in Isaiah 8:1,3. Maher-shalal-hash-baz was a son of the prophet Isaiah. The name means "In making speed to the spoil he hasteneth the prey", and was given at the moment that the king of Assyria was on his way and would soon destroy and plunder the Syrians and the Israelites. Judah would be spared due to divine intervention in response to Hezekiah's prayer for deliverance.
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God bless!
Mike,
www.thisisyourbible.com