The beisutsukai are members of a fictional society, also known as the Bayesian Conspiracy, depicted in a series of short stories by Eliezer Yudkowsky. Characters in the story include Brennan (usually the viewpoint character) and his teacher Jeffreyssai.
Derived from “beisu”, a Japanese transliteration of Bayes (though more accurate Japanese pronunciation would be bei-zu, not bei-su), plus “tsukai”, Japanese “user”, therefore “Bayes-user”. A casual, shortened pronunciation would probably come out “bayes-tskai”.
Short stories
Initiation Ceremony—Brennan is inducted into the Conspiracy
The Failures of Eld Science—Jeffreyssai explains that rationalists should be fast.
Class Project—The students are given one month to develop a theory of quantum gravity.
The Ritual—Jeffreyssai carefully undergoes a crisis of faith.