From Tolkien Gateway

One day you'll wish that you had not said that. Ape!

Apes were animals living far South of Middle-earth.

Said to live in the dark forests of the South,[2] they seem to have only been known in stories in the North. Probably due to their lesser intelligence, the word "ape", along with "maggot" and snaga, became an insult used by Orcs.[1]