In Act 3, Scene 2:
CALIBAN
(to TRINCULO) Thou liest, thou jesting monkey, thou! I would my valiant master would destroy thee. I do not lie. " (The Tempest, Act 3, Scene 2)
Caliban calls Ariel "a jesting monkey" and a liar, who is in turn, speaking as Trínculo. Ariel is imitating Trínculo´s voice and is invisible, thus the confussion. It is worth mentioning that "jester" means fool.
