couldest
could·est
(ko͝od′ĭst) or couldst (ko͝odst)aux.v. Archaic
A second person singular past tense of can1.
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.