Direct Opposites:
* Humble: This is the most direct opposite, meaning modest and unpretentious.
* Self-effacing: This implies a tendency to downplay one's achievements or importance.
Other Opposites:
* Modest: This suggests a lack of arrogance and a willingness to acknowledge one's limitations.
* Unassuming: This describes someone who is not showy or attention-seeking.
* Unpretentious: This refers to someone who is genuine and not trying to impress others.
* Subdued: This suggests a lack of fanfare or excessive enthusiasm.
The best opposite for "boastful" will depend on the specific situation and the nuances you want to convey.