There are several antonyms for the colloquial phrase "big kahuna," which generally refers to a person of great importance or influence. One opposite could be "small fry," which denotes someone who is unimportant or insignificant. Another antonym could be "underling," which suggests a subordinate position or a lack of authority. An individual who is inconspicuous or introverted might be described as the opposite of a big kahuna, as they lack the charisma and outgoing personality associated with leadership. Additionally, a newcomer or outsider could be seen as an antithesis to the big kahuna, as they do not have established connections or a history of success.