ADJ-GRADED 懦弱的;胆怯的;意志薄弱的 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. 懦弱的;胆怯的;意志薄弱的 [informal]
He would need all his authority to keep the weak-kneed volunteers from bolting.