1. A formal, serious, and sincere promise or guarantee, often made with gravity and ceremony
The president gave a solemn assurance that the funds would be used responsibly.
O presidente deu uma garantia solene de que os fundos seriam utilizados responsavelmente.
2. A solemn pledge or commitment characterized by dignity and earnestness
He offered his solemn assurance that he would keep the secret.
Ele ofereceu sua garantia solene de que manteria o segredo.