Weak-kneed

It is incontrovertible that the children grow up into rotten adults, without virility or stamina, a weak-kneed, narrow-chested, listless breed, that crumples up and goes down in the brute struggle for life.

Source: wiktionary

What these teenagers are weighing is at the very heart of Gandhian philosophy: "His absolute insistence on nonviolence, which young men see as pussyfooted and weak-kneed and — dare I say — feminine and hence not macho enough," says Guha, the biographer.

Source: wiktionary