ˈsundrop(s [f. sun n.1 + drop n.] Any of the species of Œnothera (evening primrose) which open in sunlight. 1796Nemnich Polygl.-Lex., Sundrop, Oenothera.1845–50A. H. Lincoln Lect.Bot.App. 132 Œnothera..fruticosa (shrubby œnothera, sun-drop).1856A. Gray Man.Bot. (1860) 131 Sundrops.