If you describe someone as weak-kneed, you mean that they are unable or unwilling to do anything because they are influenced by a strong emotion such as fear.
[informal]
He would need all his authority to keep the weak-kneed volunteers from bolting.
weak-kneed in British English
adjective
informal
yielding readily to force, persuasion, intimidation, etc
Derived forms
weak-kneedly (ˌweak-ˈkneedly)
adverb
weak-kneedness (ˌweak-ˈkneedness)
noun
weak-kneed in American English
(wikˌnid)
adjective
1.
having weak knees
2.
lacking in courage, determination, resistance, etc.
Examples of 'weak-kneed' in a sentence
weak-kneed
The adrenaline that had kept her so firmly in control inside the opera box was starting to bleed away, leaving her weak-kneed and worn out.
Val McDermid THE LAST TEMPTATION (2002)
Her smiles as we shyly greeted each other stunned me into delirious, weak-kneed awe.