The Pali-English Dictionary defines brahmacariya
Pali-English Dictionary wrote:a term (not in the strictly Buddhist sense) for observance of vows of holiness, particularly of chastity: good & moral living (brahmaŋ cariyaŋ brahmāṇaŋ vā cariyaŋ=brahmacariyaŋ KhA 151); esp. in Buddh. sense the moral life, holy life, religious life, as way to end suffering, Vin i.12, 19, renouncing the world, study of the Dhamma
It also says it's a compound of brahma
Brahma is defined as:
Pali-English Dictionary wrote:the supreme good; as a buddhistic term used in a sense different from the brahmanic (save in controversy with Brahmans); a state like that of Brahmā (or Brahman)
While cariya is defined as:
Pali-English Dictionary wrote:conduct, behaviour, state of, life of.