The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
intransitive verb un·der·trumped, un·der·trump·ing, un·der·trumps
To play a trump lower than another card player's trump when trump has not been led.