/ Dictionary / Index W weak sister: Meaning and Definition of Find definitions for: weak' sis'ter Pronunciation: [key] — Informal. Informal. a vacillating person; coward. a part or element that undermines the whole of something; a weak link. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. weak side weak-willed Related Content Daily Word Quiz: eureka Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language