To turn your back on the Esperanto-speaking movement and community. By connotation, someone who does this is both a quitter and a traitor to the cause. "Kabe" was originally the pseudonym of Dr. Kazimierz Bein, an early apologist for Esperanto and a writer in it, who was known for the quality of his literary works and translations. He abruptly broke all contact with the Esperanto-speaking community and left. The word can also be applied as a noun to someone who has kabe'd out.