Tattily is the adverb from 'tatty'. It means done, or more often dressed, in a shabby, worn, uncared-for way. In practice you will almost only meet it in the phrase 'tattily dressed' — someone in old, faded, cheap-looking clothes that have not been looked after. It is a mild word, not a cruel one, but it is still a judgement: it says someone did not take care with their appearance. It is quite rare, and it is British rather than American. If you want a plainer way to say it, 'shabbily' is far more common and means the same thing.