If someone is saintly, they're so perfect that they're almost too good to be true. A truly saintly person spends her life helping others.
Saintly people are extremely kind and virtuous, modeling themselves after actual saints, people who are canonized by the Catholic church after death and are believed to be in heaven. You can also use this adjective to describe things that have to do with saints, like a saintly painting or a saintly relic. Both saintly and saint are rooted in the Latin sanctus, "holy or consecrated."