Undergird is a verb that describes the act of supporting or strengthening something. There are several synonyms for undergird, including bolster, buttress, reinforce, shore up, and strengthen. Bolster and buttress both suggest the idea of providing additional support, while reinforce implies making something stronger. Shore up and strengthen both imply making something more stable or secure. Other synonyms for undergird include anchor, brace, fortify, uphold, and sustain. All of these synonyms convey the idea of providing support or strength to something, whether it be a physical structure or an abstract concept.