Groundly is an adjective that describes something that is firmly established or based on solid reasoning. Synonyms for the word include well-founded, sound, valid, rational, justifiable, reasonable, and logical. These words could be used interchangeably depending on the context. For instance, if you want to describe an argument that is supported by factual evidence, you could say that it is well-founded or sound. On the other hand, if you want to describe a decision that is based on logical reasoning, you could use the words rational, justifiable, reasonable, or logical. In summary, groundly is a versatile word that has several synonyms that are useful for expressing the same concept in different ways.