The phrase "in full feather" refers to someone or something that is at its highest point of greatness or peak performance. Some synonyms for this phrase include "in peak form," "at the top of its game," "in top shape," "performing at its best," and "in prime condition." These synonyms can be used to describe a variety of situations, from a sports team that is winning every game to a person who is excelling in their career. Whatever the context may be, the phrase "in full feather" and its synonyms indicate a high level of excellence and achievement.