Used when something is so clearly true or so clearly good that no reasonable person could argue about it. You use it to shut a door: there is nothing here to discuss. It attaches most often to qualities and facts that are being praised or defended — his unquestionable talent, her unquestionable honesty, the unquestionable success of the plan, unquestionable proof. Notice that it is usually a strong compliment or a strong claim, not a neutral description. It is fairly formal; in daily speech people say 'there's no doubt about it' or 'nobody can argue with that'. There is a useful pairing to learn here. 'Questionable' means doubtful, suspicious, probably not good — questionable behaviour, a questionable decision. 'Unquestionable' is its opposite: beyond doubt. Both words are really about whether people can raise questions, but they landed in opposite places, and 'questionable' picked up a disapproving flavour along the way. One honest thing to add: 'unquestionable' says nobody CAN reasonably question it, but people often use it about things that are in fact arguable, simply to sound certain. Politicians and advertisements are fond of it. So when you meet it in the wild, it is worth asking yourself quietly whether the thing is really beyond question, or whether the writer just wants you to stop asking.