Question: what episode does finn lose his arm?
Answer: In the Season 6 premiere "Escape From the Citadel", Finn has lost half of his right arm, from which a flower protruded. He later regains his arm, now as a prosthetic, in the episode "Breezy", only to lose it again in the episode "Reboot" when his grass arm transforms itself into a humanoid, sentient being.