Odysseus tells Polyphemus his name is "Nobody" to prevent Polyphemus from being able to successfully call for help when Odysseus enacts his escape plan.
When Odysseus and his men are trapped in Polyphemus's cave, he's horrified as the cyclops eats some of his men. To escape, he sharpens a large wooden club and then he convinces Polyphemus to get drunk. Odysseus relates the story, saying, "Three times I poured some out and gave it to him, and, like a fool, he swilled it down."
When Polyphemus is drunk, Odysseus claims his name is Nobody.
