Underpin is a term used in construction and engineering to refer to the process of providing additional SUPPORT to an existing structure. It involves adding extra SUPPORT to the foundation of a building or other structure to make it more stable and secure. This can be done by adding additional materials such as concrete, steel, or timber, or by using techniques such as underpinning or underpinning piles. The term 'underpin' is also used to refer to the process of providing additional SUPPORT to a structure that has already been built. In this case, the additional SUPPORT is usually provided by adding additional materials such as concrete, steel, or timber, or by using techniques such as underpinning or underpinning piles.