Expressing one's views passionately or self-importantly, as in Dexter can't resist getting on his soapbox about school expenditures. This expression comes from the literal use of a soapbox as an improvised platform for a speaker, usually outdoors. [Mid-1600s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.