Yellowbelly is a colloquial term used to describe someone who lacks courage or easily gives in to fear. Some synonyms for this word can include "coward," "chicken," "wimp," "scaredy-cat," "timid," "spineless," and "weakling." Each of these words also carries a connotation of being afraid or lacking strength. However, it is important to use these words with caution, as they can be hurtful or derogatory. In certain circumstances, it may be more appropriate to use words such as "cautious," "prudent," or "careful" to describe someone who is not willing to take risks. It's always important to consider context and tone when using synonyms for any word.