RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Rune skirt
The Old School RuneScape Wiki also has an article on Rune skirt.

The Rune skirt is a plated skirt that can be made at level 99 Smithing using 3 runite bars, yielding 225 experience. It requires level 40 Defense to equip.

It can be exchanged for Rune Plate Mail Legs from Thrander in southeast Varrock, or purchased from Scavvo's Rune Shop in Champions' Guild.