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.
"Nu, Johano ne plu ĉeestos niajn kunvenojn. Li verŝajne kabeis antaŭ du semajnoj."
"Well, John won't be attending our meetings anymore. He apparently kabe'd out two weeks ago."