The phrase "hollow man" can describe someone who lacks genuineness or who seems to be empty inside. There are many synonyms for this term that can help to convey this idea, such as insincere, phony, fake, shallow, or superficial. Other words that could be used to describe an individual who is a "hollow man" might include disingenuous, deceptive, or even two-faced. These terms all express a sense of being dishonest or inauthentic in some way. Ultimately, any of these synonyms could be used to paint a picture of someone who is not true to themselves or their values, but rather is simply putting on a false front in order to impress others.