Filling yarn is a type of yarn used in weaving. It is also known as WEFT, which is a four-letter word that can be found in crosswords. Filling yarn is used to create the widthwise threads in a woven fabric, and it is usually made from the same material as the warp yarn. The filling yarn is inserted between the warp yarns, and it is held in place by the warp yarns. Filling yarn is an important part of the weaving process, as it helps to create the desired pattern and texture of the fabric.