Unable to answer, because you truly cannot think of anything. If a question stumps you, your mind goes blank — you are not refusing to answer and you are not being lazy; nothing comes. You say 'I'm stumped' when a puzzle beats you, when a student asks something you never thought about, when a machine breaks and no one can see why. It is warm and casual, common in everyday speech, and it carries no shame at all — grown-ups say it about themselves happily, and doctors and engineers use it about hard cases. Two patterns matter. 'Be stumped' (I'm stumped, the doctors were stumped) is the state. 'Stump someone' is the verb: the question stumped me. It is usually about one specific problem, not about life in general — you are stumped by a question, not stumped about who you are. The picture behind it is a tree stump left in a field: you were walking along, and something short and stubborn stopped you.