A suppliant is someone who begs or prays for something, and to be suppliant is to humbly pray or ask for forgiveness. Puppy-dog eyes can help if you’re trying to look suppliant.
Like the related word supplicant, suppliant has do with praying in a humble manner. A suppliant student in the principal's office is going to have better luck avoiding detention than a disrespectful one. Suppliant is not a word you hear much these days — it was tailor-made to be used in the days when people spent much of their lives on their knees in big stone buildings begging for things from monarchs or from God.