Most junior refers to the least experienced or the newest member of a group or organization. The antonyms for the phrase "most junior" are "senior," "experienced," "veteran," "seasoned," "mature," and "established." Senior, experienced, and veteran represent the opposite of being new or inexperienced, whereas seasoned and established indicate having been in a position or role for a considerable period. Opposing the least or most junior would be the highest or most senior and experienced positions in a hierarchy. These antonyms are crucial to understand when discussing structure or ranks within a group or organization to avoid any confusion or ambiguity.