Definition of "shillelagh"

[T]he stem of a stout young oak or ash tree, into the end of which, where the roots had been rounded off, a quantity of molten lead was poured, making the shillelagh more formidable in such hands than a sword would have been – much harder to parry, and impossible to break.

1842, Charlotte Elizabeth, “Letter IX. The Dumb Boy.”, in Personal Recollections. [...] From the London Edition, New York, N.Y.: John S. Taylor & Co. No. 145 Nassau-Street, Brick Church Chapel, pages 151–152

It beats all your guns and your rifles, / For it goes off whene'er you desire, / And it's sure to hit just what it aims at, / For shillelaghs they never miss fire.

, T. Maclagan, “The Twig of the Shannon”, in Maclagan's Musical Age Songster, Containing All the Most Popular Songs Sung by T. Maclagan, London: The Music-Publishing Company, 19 Peter's Hill, St. Paul's, E.C., page 27

"My sorrow!" cried Toirdealbhach. "What do I want to be a saint for at all, is my puzzle! If I could fetch one crack at somebody with me ould shillelagh"—here he produced a frightful-looking weapon from under his gown—"wouldn't it be better than all the saints in Ireland?"

1958, Terence Hanbury White, The Once and Future King, London: William Collins, Sons

The precise size and shape of a shillelagh can be hard to agree upon, but most of those who have an interest in Irish culture will readily acknowledge that a knobbed stick, made of oak or blackthorn, are the distinguishing characteristics of a shillelagh.

2007, John W. Hurley, “Irish Martial Arts”, in Shillelagh: The Irish Fighting Stick, Pipersville, Pa.: Caravat Press, page 15