tone·less (t?n?l?s)adj.1. Lacking tone.2. Lacking vitality; listless.tone?less·ly adv.tone?less·ness n.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.