Why Arjuna is called Kiriti (किरीटी)?
The word किरीटी is derived from किरीटिन् – [किरीट-इनि] – one who wears a crown or diadem
किरीटी – किरीटः अस्ति अस्य इति किरीटी
It is mentioned by Arjuna himself in the Mahabharata, Virat Parva, chapter 44 verse 17
पुरा शक्रेण मे दत्तं युध्यतो दानवर्षभैः |
किरीटं मूर्ध्नि सूर्याभं तेनाहुर्मां किरीटिनम् ||
In the previous times, when I was fighting with danavas (दानव) that time Lord Indra put a crown on my head, it was shining like the sun. It is because of the same I am called as Kiriti (किरीटी )