The word "rbf" appears to be a typographical error or an abbreviation that is not commonly recognized in English. Based on common usage, it is likely you meant "RBF," which stands for "Resting Bitch Face." This term refers to a facial expression that unintentionally appears unfriendly, annoyed, or uninterested, even when a person is not feeling that way.
The term "Resting Bitch Face" first appeared in popular culture around the early 2000s, particularly in the context of social media and online discussions. The phrase became widely known after a 2013 interview with actress Anne Hathaway, who humorously referred to her own RBF.