When Lord Hanuman was a child, he mistook the Sun as a ripe mango and flew into the space to consume it. When he swallowed the sun, there was darkness everywhere. To save the world, Lord Indra (king of the Gods) came to rescue the Sun and used his weapon Vajra on Hanuman. Vajra hit Hanuman on his jaw (Hanu in Sanskrit), thus, disfiguring it. Hence, he got the name Hanuman which means one with a disfigured jaw.