Ae·a·cus (???-k?s)n. Greek Mythology The first king of Aegina, known for his piety and justice, appointed as a judge in Hades after his death.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.