From Longman Dictionary of Contemporary Englishweak-kneedˌweak-ˈkneed adjective informal BRAVEDECIDElacking courage and unable to make your own decisions
Examples from the Corpus
weak-kneed• For many years Jack Benny had a comic character on his programs named Mr Shlepperman-a timid, weak-kneed character.• a weak-kneed coward• The other two, a couple of weak-kneed cowards, had thought that was enough and run off.• Criticisms of solipsism should therefore count as criticisms of the weak-kneed scepticism embodied in foundationalism.• He stood there like a weak-kneed schoolboy who'd impulsively confessed to stealing the headmaster's wallet. 