The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
yeu-
DEFINITION:
Vital force, youthful vigor. Oldest form *2yeu-; variant of aiw-. Suffixed zero-grade form *yuwen- (< *yu-en-), “possessing youthful vigor,” young. 1. Further suffixed form *yuw-ti-.youth, from Old English geoguth, youth, from Germanic *jugunthi-,*jugunth.2. Further suffixed form *yuw-ko-.a. (i)young, from Old English geong, young; (ii)junker, from Old High German junc, young; (iii)younker, from Middle Dutch jonc, young. (i)–(iii) all from Germanic *jungaz, from *juwungaz;b.gallowglass, from Old Irish óac, from Celtic *yowanko-.3.junior, June, Juno, juvenile; rejuvenate, from Latin iuvenis, young. (Pokorny 3. eu- 510.)
2yeu-; variant of aiw-. Suffixed zero-grade form *yuwen- (< *yu-
-ti-. youth, from Old English geoguth, youth, from Germanic *jugunthi-, *jugunth
. 2. Further suffixed form *yuw
eu- 510.) 
