a vacillating person; coward.
a part or element that undermines the whole of something; a weak link.
Words nearby weak sister
Origin of weak sister
First recorded in 1855–60
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for weak sister
US informal a person in a group who is regarded as weak or unreliable
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012