The Tremendous

On Bilbo Baggins' first secret exploration of the Lonely Mountain he had found Smaug asleep, and successfully carried away a single cup from the Dragon's vast treasure. On his second attempt, Smaug appeared to be asleep once again, but it soon became clear that this was a ruse, and that the Dragon was watching for the burglar's return.

Bilbo attempted to distract the Dragon with praise, claiming to have come to Erebor to gaze on the magnificent creature, and giving Smaug a string of flattering titles. He started his speech, 'O Smaug the Tremendous'¹, a choice of words that mollified the Dragon a little, and opened a perilous conversation that led to Bilbo's discovery of Smaug's weak spot - a discovery that would ultimately bring about the Dragon's doom.

It's perfectly possible that Bilbo's choice of title was a spontaneous one, but he was perhaps influenced by his unwilling visit to Goblin-town earlier in his adventures. At one point there, the Great Goblin was addressed by one of his people as 'O truly tremendous one!'² This may very well be simple coincidence, but the initial 'O' in both cases hints that Bilbo might have remembered this previous encounter and later used the same title himself.