If someone is boastful, they talk too proudly about themselves — their money, their grades, their cleverness — as if they need you to be impressed. A little pride can be fine; boastful means they keep pushing it on other people. It is a criticising word. You would not smile and call a friend boastful unless you thought they had gone too far. It can describe a person or the way they talk: a boastful story, a boastful smile. It is a normal word, not slang, and a little more formal than show-off. Though actually, people sometimes boast without meaning to sound unkind; boastful is how it lands on the listener when there is too much of it.