The way in which one person would greet another person named "Fi".
Also (erroneously) short for "high fidelity" - used to describe recorded sound that sounds faithfully like the original production of the sound. It's a term that originated in the 1950's (before stereos) to describe sound-recording equipment that provided faithful reproductions, but nowadays is used mostly by audiophiles.
Fi: Hey there buddy!
Pedophile: Come over here kiddie, these Bose speakers produce such high-fidelity sound!
Audiophile: No they don't.