According to Commentary by Sri Parasara Bhattar on Vishnu Sahasranamam
- Sahiṣṇuḥ - He translates this as the One who forgives
Bhagavan Vishnu is called Sahiṣṇuḥ because its his nature(denoted by suffix "Ishnuch") to forgive the innumerable sins of people when they become well disposed towards him.All accumulated sins which have been committed before or are about to be committed later, all sins done consciously or unconsciously by all organs of sense at all times in all ways, sins committed by not following injunctions or doing acts forbidden by Sastras, sins common and uncommon, sins committed by abusing and insulting him and sins committed by his own devotees, all are forgiven and tolerated by the Lord.To the Lord forgiving is like a natural quality just like coolness is a natural quality to moon and water.
I found another beautiful commentary by Dvaita Scholar Sri T.S. Raghavendran which explains the name in the following manner:
- Lord Vishnu is called Sahiṣṇuḥ because He tolerates the
offences committed by His devotees.
The amount of tolerance that Lord Vishnu has,
nobody else can even think of this quality.This is because Vishnu alone has the capacity to do anything, not to do anything, to do anything in any different way.Still Paramatma tolerates every offence and does only according to Vedic mandates and injunctions.This is to save the validity of the Vedas, because when the validity of the Vedas is protected then dharma and adharmas are saved in the Jagat and then only there
will be a cultured society, peace, etc., in the world.So
Paramatma, although He is not bound to any rules and regulations, and is totally beyond and above them, still out of of tolerance does as per Vedic Vakyas. He is the best सहिष्णुः ॥