It comes from the Hebrew verb "mashach", which simply means to annoint as with oil, which was a common practice for kings, but also for other offices, and even objects. The participle form, "meshiach," from which we get the term Messiah, is used of kings for sure, but also of priests and prophets, which is of interest since Christ is not only king, but also priest (of the order of Melchizedek) and prophet.