Use the word tatterdemalion for anyone who's dressed in messy, ragged clothes. You may want to call your brother a tatterdemalion if he shows up for Thanksgiving dinner wearing a dirty sweatshirt and his favorite ripped jeans.
A slightly more common word for a tatterdemalion is a ragamuffin. As an adjective, tatterdemalion means "unkempt" or "shoddy," so go ahead and ask your brother, "Why are you wearing that tatterdemalion outfit?" While the tatter part of the word is fairly clear (a tatterdemalion's clothing is in tatters), the ending is a mystery. There may be a connection to the Frenchde maillot, or "shirt."