caitheamh Definitions (by source) Show all Leabhar Branach: The Book of the O'Byrnes act of spending, consuming. do ch. a rosc, i.e. to shed all the tears in their eyes, 1553. Leabhar Branach: The Book of the O'Byrnes. Editor: Seán Mac Airt. Poems on the O'Reillys spending v.n. of caithim. go mbí an rath i mbun caithmhe (proverbial), 1204 Poems on the O'Reillys. Editor: James Carney.