He was the nephew of the elder Saṅgharakkhita, hence his name.

Once, at a certain monastery he was given two sets of robes, and immediately put away the better set to be given as a present to his uncle and teacher. At the end of the rains, he went to the monastery of his uncle, and, having waited upon him, offered his gift. However, his uncle refused it in spite of the youth’s repeated request. Thereupon the nephew became sulky, and while fanning the elder, started to think what he could do if he became a layman. He would sell his robe, buy a she goat, get rich thereby, marry, and have a son. While taking the child out for a walk, he would ask to be allowed to carry him, his wife would refuse and carry him herself. Then she would drop the child and her husband would hit her. So thinking, in his absent-mindedness, the youth hit the elder with the fan. The elder read his thoughts and rebuked him. The youth then started to run away, but the monks caught him and brought him before the Buddha. The Buddha taught him on the difficulty of keeping the mind in check. At the end of the discourse the youth became a Stream-winner. DhA.i.300 ff.