There are plenty of synonyms for "pencil-thin," which is widely used for describing someone or something that is very thin. Some of these synonyms include thin as a rail, skinny, bony, gaunt, emaciated, scrawny, lanky, gangly, spindly, and slender. Each of these words can be used in the same context as "pencil-thin," depending on the situation. For instance, "slender" can be used to describe something that is thin yet graceful, while "gaunt" might be used to describe someone who looks thin and tired. These synonyms help to convey the same meaning as "pencil-thin" while giving the reader or listener a more interesting and varied choice of words.