The term PESTLE'S PARTNER is referring to the job description of MORTAR, which is a 6 letter word. MORTAR is a material used in construction that is made from a mixture of sand, cement, and water. It is used to fill gaps between bricks and stones, and to bind them together. MORTAR is an essential part of masonry work, and is often used in combination with a pestle to create a strong bond between the materials.