/ Thesaurus / F Synonyms of fast of the firstborn Find synonyms for: Noun1. Fast of the Firstborn, minor fast dayusage: (Judaism) a minor fast day on Nissan 14 that is observed only by firstborn males; it is observed on the day before PassoverWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: dormant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language