Ian McKellen -- Gandalf's Protecting My Ass

Ian McKellen has magic underwear ... and yesterday, he showed off his special tighty-whities during a theater performance in New York City.

Of course, the face protecting Ian's pooper is Gandalf from "Lord of the Rings" -- the character played by Ian in the LOTR movies.

Fun Fact -- the undies were actually a gift from "Watch What Happens Live" host Andy Cohen.

Mazel.