The Táin, episode 12 – Bring it!
Read this story from the first episode here.
In the original legend, Finn McCool knows that he cannot defeat Benandonner, and so he asks his wife for help. She dresses him as a baby, and when the giant comes to the door, she plays a trick on him. She tells him that Finn is out, but will return shortly. She offers a variety of falsehoods (such as pointing to a heavy oak door and claiming it is Finn’s shield, and a pine tree and claiming it is his spear), and finally introduces him to Finn’s newborn. When the giant sees that Finn’s child looks like a huge, hairy man, he is scared of what size Finn must be, and opts not to fight.