oak tag or oak·tag (?k?t?g?)n. See tagboard.[Perhaps from its color, which originally resembled that of the oak boards used in book bindings until the 16th century, or perhaps from oakum board, a kind of sturdy paperboard made from oakum.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.