(abbreviation, alt-of, countable) Abbreviation of radial basis function, a type of equation used to approximate a vector field, used in neural networks.
"When I first met you I was scared of you."
This phrase is commonplace for people with an RBF. They come to accept the natural state of their face as one that exudes feelings of anger, disgust or misery. 2013 November 8, Caren Duane, “This Is Just What I Look Like”, in The Dartmouth Mirror, Dartmouth College, page 8:
If you're looking for a more positive outlook on your RBF, here are a few things to consider: solicitors on campus are less likely to hound you with flyers, you may be less prone to smile wrinkles, and you probably have a very effective poker face. 2015 April 27, Angie Burkhart, “For some, smiling is too much work”, in The Coyote Chronicle, volume 52, number 2, San Bernardino: California State University, page 8:
Her RBF has been with her ever since she was a baby, forcing her to fake a smile all the time so people would stop questioning her. 2014, Andie Leeds, "Sass and the Milly", The Colonnade (Georgia College & State University), 10 October 2014, page 11