Lacking in courage, determination, resistance, etc.
Webster's New World
(literally) Used other than as an idiom: seeweek,"Ž kneed.
Weak-kneed Sentence Examples
What is it that makes us go weak-kneed every time we see another example of our favorite car on the road?
The Socialist party, which had grown powerful under a series of weak-kneed administrations, now began to show signs of division; on the one hand there was the revolutionary wing, led by Signor Enrico Fern, the Mantuan deputy, which advocated a policy of uncompromising class warfare, and on the other the riformisti, or moderate Socialists, led by Signor Filippo Turati, deputy for Milan, who adopted a more conciliatory attitude and were ready to ally themselves with other parliamentary parties.
The medicine is so simple in application and so easily available that it is served out almost automatically and indifferently, to every law-breaker; the pickpocket and the burglar are locked up next door to the clergyman at variance with his bishop; the weak-kneed and self-indulgent drunkard rubs shoulders with the political zealot who has endangered the peace of nations.
She'd had never felt overwhelmed by a man before, and she'd certainly never been a woman who felt weak-kneed!