Your Magnificence

When Bilbo Baggins and the Dwarves discovered the hidden Dwarf-door in the side of Erebor, Bilbo crept down the secret passageway into Smaug's treasure chamber. Finding the Dragon asleep, he carried away a golden cup. This delighted the Dwarves, but infuriated Smaug, who immediately noticed the theft and launched a fiery attack against the mountainside before returning to his deep halls.

Bilbo resolved to make another visit to the Dragon in the hope of finding a weakness. Timing his trip to the middle of the day, when Smaug would presumably be asleep once again, he used his Magic Ring to slip invisibly back down the tunnel to the Dragon's lair. Smaug did indeed appear to be sleeping on his bed of gold, but this was a ruse; he was waiting for the thief to visit his hoard again. Because the Ring made Bilbo invisible, he was able to elude Smaug's glare, and instead the two bantered for a time, with Bilbo using ridiculous flattery and riddling talk to avoid directly insulting the monster. Eventually he discovered what he had hoped to find: the Dragon revealed a bare unarmoured patch under its left breast, and as soon as he had spotted this, Bilbo hastily took his leave. In doing so he addressed the Dragon - somewhat mockingly - as 'Your Magnificence' before hurrying away, followed by a gout of flame.

Bilbo's news proved instrumental in bringing about the Dragon's downfall, but not directly. As he told the Dwarves what he had discovered, he was overheard by a watching thrush. This bird carried news of the bare patch on Smaug's underside to Bard in Lake-town, and so Bard's Black Arrow was able to bring an end to the fearsome Dragon of Erebor.