A wuss is someone you think is weak, soft, or too easily frightened — the person who will not go on the roller coaster, who cries at a small cut, who backs down from an argument. It is an insult, but usually a light one, said between friends: 'Come on, don't be a wuss, the water's fine.' It is American slang and quite informal. Do not use it in a school essay, in an email to a boss, or to a stranger. There is also a verb, 'wuss out', which means to promise to do something and then get scared and not do it: 'He said he'd sing, then he wussed out.' The verb is actually more common now than the noun. One thing worth knowing: because the insult means 'weak', it carries an old idea that being frightened or gentle is unmanly, and quite a lot of people dislike it for that reason. It is much milder than the harsher insults with the same meaning, so it will not usually start a fight, but among careful speakers it can still land badly. Between close friends teasing each other, it is fine. Anywhere else, there are safer words.