Pain-ties are special punishment underwear for girls in Lurking Dragon's Rejuve Universe. The corresponding garment for boys are bee-riefs. Such punishment underwear see primary usage with Penitatas children however might find use with particartly naughty rejuves of other types.
Here is how they are described in the first of the Melody stories (Melody's Xmas):
Next was Granma's present. Melody opened the box to find a full set of four "Pain-ties™" These thickish little-girl panties were specifically designed for well-spanked bottoms. Have you ever been stuck by the end of a nylon thread in your clothing?? Pain-ties were DESIGNED to have such threads in them...in the area covering the bottom. These were called 'biters' colloquially. The first pair had only a few such threads. They were the thinnest pair, looking like regular plain yellow cotton panties. But when worn, every so often a thread in the bottom area would stick, causing a brief fiery sting. Sitting on them, (in the corner, for example), would cause more frequent stings as the child's own weight pressed the special fabric into her bottom. The next pair were pink, covered in a pattern of small hairbrushes and paddles. These were small enough to only be recognizable at close range, so the panties could be worn without being TOO obvious. It held about twice the density of 'biters' as the first pair, resulting in more frequent sticks. The third pair was white with a pattern of rosy red bare bottoms on it. These were designed to be obvious; even if worn under a short skirt, anybody could tell a child was wearing Pain-ties. The density of biters was about the same as the yellow pair, but the threads were designed to be a bit longer, imparting a fiercer sting. The last pair weren't really panties at all, as they extended right down the thighs halfway to the knee. The back of these panties had a single design; a well spanked bare bottom painted right over the appropriate area, with the spanking carried on down both legs. These would be obvious under ANY kind of dress less than knee length. The density of 'biters' (type 2) was higher, too, and covered not only the bottom but also the upper thighs.
Standing while wearing any pair of Pain-ties was an uncomfortable experience. Sitting down in them was much worse. Add in a well- spanked bottom, and corner time, even standing, becomes quite an ordeal. And when a well-spanked naughty child has to SIT in the corner wearing Pain-ties.......