Treva is almost cetainly Celtic in origin, and is thought to be the feminine form of the boy's name Trevor. Trevor or Trefor is Welsh in origin, and derives from a well known place name - several places in Wales are called Trefor. The meaning comes from tref, "settlement" and for, derived from mawr, which means "large" - so, basically, Trevor means "large settlement." The name is often shortened to Trev.
It is also possible that Treva is a feminine form of the old Cornish boy's name Trevelyan. This is also a place name, meaning "settlement of Elian." Settlement in Old Cornish is Tref, as in Welsh; Elian is an old Celtic name, but its original meaning is unclear. Oddly enough the name Elvis may originally have been Elian - there was an early Christian saint who was known by both names.
It is also possible that Treva is a feminine form of the old Cornish boy's name Trevelyan. This is also a place name, meaning "settlement of Elian." Settlement in Old Cornish is Tref, as in Welsh; Elian is an old Celtic name, but its original meaning is unclear. Oddly enough the name Elvis may originally have been Elian - there was an early Christian saint who was known by both names.