Know-it-all (noun)

1. A person who acts as if they know everything and refuses to accept advice or information from others.
2. A person who is irritatingly confident in their own knowledge or expertise.

Origin:

Compound of know + it + all.

Examples:

1. He's such a know-it-all, he won't even listen to other people's opinions.
2. The know-it-all attitude of the new employee was off-putting to her colleagues.