"Bosom buddy" is a term used to describe a close friend, someone you confide in and share intimate moments with. However, there are several antonyms to this phrase that differentiate the levels of friendship. Some possible antonyms could include "acquaintance," "stranger," "opponent," "enemy," or "rival." While an acquaintance might be someone you know casually, a stranger is someone you have no personal relationship with. An opponent or enemy, on the other hand, are people you are actively against or have negative feelings towards. Finally, a rival could be someone you are competing with or trying to outdo, which would suggest a more tense and competitive relationship.