Like many first names, it comes from a last name. Llewellyn is a well established surname in Wales. One of Shakespeare's most famous characters had this name: He appears in Henry V and is called "Fluellen" because of the Welsh pronunciation. The double-l at the beginning is pronounced with a blowing of air around the tongue, somewhat similar to an "F" sound.
Since it's derived from a family name, you may consider it applicable to either sex, but it has been used more for boys than for girls. Of course, Ashley was mostly a man's name until recently, and has become almost entirely female now. Taylor and Logan, both traditional male names, are being used for more and more girls.