The phrase "most whiteheaded" refers to someone or something that has a predominantly white head, hair, or feathers. There are several synonyms you can use instead of this phrase, depending on the context. For example, other ways to describe the same thing include "very pale-headed," "mostly ivory-haired," "almost completely white-haired," "mostly alabaster-feathered," or "largely snowy-crowned." Each of these descriptions conveys a similar meaning and can be used depending on the specific context of the situation. Overall, using a variety of synonyms can help you add depth and variety to your writing.