Undergirds is a word that refers to the support or foundation upon which something is built. There are several synonyms for this term, including buttresses, reinforces, bolsters, strengthens, sustains, and secures. These words all convey the same meaning of providing a firm and solid base for something to be built upon or supported by. For example, the teacher's encouragement and positive feedback buttressed the student's confidence, and the steel beams reinforced the structure of the building. Whether describing a physical or metaphorical foundation, these synonyms help to express the importance of structural support and stability.