The verb "kabei" means to leave the Esperanto community. What is the source of this word?
It comes from the pseudonym of Kazimierz Bein.
He was also, for a time, a prominent Esperanto author, translator and activist, until in 1911 he suddenly, without explanation, abandoned the Esperanto movement. Bein became at least as well known for his involvement with Esperanto as for his medical accomplishments, and as much for the manner in which he left the Esperanto movement as for what he had accomplished within it. Among Esperantists, he is commonly known by his pseudonym, Kabe.
(Emphasis is mine.)
It comes from Kazimierz Bein aka Kabe, one of the early Esperantists, who all of a sudden in 1911 stopped all his Esperanto activity without any comment.