ill-treat (?l?tr?t?)tr.v. ill-treat·ed, ill-treat·ing, ill-treats To treat unkindly or harshly. See Synonyms at abuse.ill?-treat?ment 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.