A doofus is a dummy or a simpleton — in other words, a goofy person who's not very smart. You might decide not to buy a top hat and suspenders after trying them on, looking in a mirror, and realizing you look like a doofus.
Use the informal doofus when someone acts with stupidity or does something ridiculous and unthinking. If your brother angrily accuses you of stealing his favorite sunglasses, you can say, "You mean the ones on your head, doofus?" The word was originally 1960s US student slang, and it's thought to be modeled after goofus, an older term with a similar meaning, or to stem from the Scots doof, or "dolt."