/ Dictionary / Index T thunder egg: Meaning and Definition of Find definitions for: thun'der egg" Pronunciation: [key] — Geol. Geol. a globular concretion of opal, agate, or chalcedony weathered out of tuff or basalt. Cf. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. thundercloud thunderhead Related Content Daily Word Quiz: morose Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language