Hanuman's mother was Anjana, but when he was born, she left him alone in the forest.
Baby Hanuman grew hungry. Very very hungry.
So when he saw what he thought was a mango in the sky, he wanted to eat that mango. He flew into the sky, eagerly reaching with his hands to grab the mango, ready to bite it with his teeth.
But it wasn't a mango. It was the sun!
Indra the storm-god grew angry and hurled a thunderbolt at Hanuman. Hanuman fell to the ground and broke his jaw.
That's how he got the name Hanuman, Big-Jaw.
Inspired by: The Ramayana (Uttara Kanda, 35).
Notes: The complete story involves the eclipse-demon Rahu who is angry that Hanuman might get to eat the sun, but I did not have room to include Rahu here. Jambavan also tells this story directly to Hanuman in sarga 66 of the Kishkindha Kanda.