A twit is a foolish person. (It's also not any kind of goldfish.) It's most often used in the expression "to feel a bit of a twit". Another common usage is as a substitute for the word "twat" when you realise mid-sentence that you are talking to a vicar / primary school teacher / the Pope / etc.

Twit is mainly used today for its comedy value, as it is possibly the most ineffectual insult in the English language.

Roald Dahl once wrote a book called The Twits, which painted some people with the surname Twit in a very bad light.