From egg on + -er.
egger-on (plural eggers-on)
- Agent noun of egg on: one who eggs on, one who encourages or provokes another.
1766 January, William Smith, Mirror[2], manuscript, page 4:Consult the mirror see it shews / Your Eggers on will turn your foes
2006, James M. Lawson, Jr., “Higher Ground: The Nonviolence Imperative”, in Peter Laarman, editor, Getting on Message: Challenging the Christian Right from the Heart of the Gospel, →ISBN, page 27:I no longer hesitated; the next day I spent the lunch hour fighting the older, bigger, and heavier boy. But the eggers-on were disappointed because I was not beaten, despite the headache I carried away.
2008, Alexander Humez, Nicholas Humez, On the Dot: The Speck That Changed the World, →ISBN, page 163:Our heartfelt thanks go out to the goodly fellowship of personal sources, expert witnesses, and eggers-on who have supplied us with many of the cheerful facts and enlightening suggestions that have gone into the making of this book […]