The Buddha explains why Venerable Pilindavaccha addresses the bhikkhus with the term 'outcaste'.
Pilindavaccha sutta - Pilindavaccha
Thus have I heard — At one time, the Blessed One was dwelling in Rājagaha, in the Bamboo Grove, the Squirrels' feeding ground. Now, at that time, venerable Pilindavaccha was addressing the bhikkhus with the term 'outcaste'. Then, several bhikkhus approached the Blessed One; having approached, they paid homage to the Blessed One and sat to one side. While seated to one side, those bhikkhus said this to the Blessed One: "Venerable sir, venerable Pilindavaccha is addressing the bhikkhus with the term 'outcaste'."
Then the Blessed One addressed a certain bhikkhu: "Come, bhikkhu, go to Venerable Pilindavaccha on my behalf and address him, saying: 'Friend Pilindavaccha, the Teacher calls you.'" "Yes, venerable sir," replied that bhikkhu to the Blessed One, and, having answered, he approached Venerable Pilindavaccha; having approached, he said to Venerable Pilindavaccha, "Friend Pilindavaccha, the Teacher calls you."
"Yes, friend," replied Venerable Pilindavaccha to that bhikkhu, and, having answered, he approached the Blessed One; having approached, he paid homage to the Blessed One and sat down to one side. While seated to one side, the Blessed One said to Venerable Pilindavaccha: "Is it true, Vaccha, that you are addressing the bhikkhus with the term 'outcaste'?" "Yes, venerable sir," he replied.
Then the Blessed One, reflecting on Venerable Pilindavaccha's past lives, addressed the bhikkhus: "Bhikkhus, do not find fault with Venerable Vaccha. Bhikkhus, Vaccha does not have any inner defilement that causes him to address the bhikkhus with the term 'outcaste.' For five hundred consecutive lifetimes, Venerable Vaccha was born in a family of brahmins. Thus, this habit of using the term 'outcaste' has long been practiced by him. Therefore, Vaccha addresses the bhikkhus with the term 'outcaste'."
Then the Blessed One, having understood the significance of it, on that occasion exclaimed:
"In whom there is no deception and no conceit, who is without craving, free from 'mine', and fulfilled; Who has dispelled anger, with completely cooled mind, they are a brahmin, an ascetic, and a bhikkhu."