Utterly means completely, totally, one hundred percent — with nothing left over. It is an intensifier: its job is to make the word after it stronger. But it does not go with just any word, and that is the real lesson here. Two rules that native speakers follow without thinking. First, utterly nearly always attaches to something bad or extreme: utterly exhausted, utterly ridiculous, utterly destroyed, utterly useless, utterly miserable, utterly wrong, utterly alone. You do not say 'utterly happy' or 'utterly delicious' — those sound strange, and a listener would smile. If the feeling is very strong and dark, or the failure is total, utterly fits. Second, it goes with words that are already absolute or extreme, not with ordinary middle-of-the-road ones: 'utterly impossible' works, 'utterly difficult' does not. There is one warm exception you will hear a lot: 'utterly delighted' and 'utterly charming' — very strong positive feelings, usually in polite or slightly posh English. In grammar it sits before an adjective, a past participle, or sometimes a verb: 'She utterly refused.' 'The town was utterly destroyed.' In register it is a shade more literary and emphatic than 'completely'; it is common in writing and in emotional speech, and it makes a sentence sound like you mean it. Close friends: completely, absolutely, totally, entirely — with 'totally' being the casual one young people use, and 'utterly' the one that sounds like a novel or a person genuinely upset.