the third last syllable in a word, as -lu-
Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by
Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'antepenult'contr. < L (syllaba) antepaenultima < ante-, before + paenultima, the last but one < paene, almost + fem. of ultimus, last