a person in a group who is regarded as weak or unreliable
weak sister in American English
US, Slang
one who is cowardly, unreliable, etc.
weak sister in American English
noun informal
1.
a vacillating person; coward
2.
a part or element that undermines the whole of something; a weak link
Word origin
[1855–60]This word is first recorded in the period 1855–60. Other words that entered Englishat around the same time include: kickoff, lavabo, output, pickup, specialist