The antonyms for the word "undermined" are "support" and "strengthen." When something is undermined, it means that its stability, strength, or position has been weakened or compromised. To support, on the other hand, is to provide assistance, backing, or endorsement to something or someone. To strengthen is to increase the durability, power, or effectiveness of something. Using these antonyms in a sentence, we could say that the government's policies are undermining the economy, but with effective measures, we can support and strengthen it. It is important to choose our actions carefully, so that we do not undermine what we aim to support and strengthen.