Your will is the part of you that decides and then holds on. So a weak-willed person is someone who meant to do the hard thing and did not do it: they planned to study and watched videos instead, planned to save money and spent it, planned to stop smoking and smoked. The failure is not in knowing what is right — they know perfectly well — it is in keeping going when the easy choice is right there. It is a criticism, but a mild and rather sympathetic one, because everybody understands the feeling; people often say it about themselves, half-laughing: 'I'm too weak-willed to keep biscuits in the house.' Its opposite is strong-willed, though that one has drifted — strong-willed usually means stubborn, especially about children, rather than admirably determined. Weak-willed is nearly always used of people and their choices, not of things. Compare weak-kneed, which is about courage under pressure from other people, and weak-minded, which is about not thinking for yourself.