* A profession: It would have been clear that the person was involved in shipping and trade on the lake. "Barrel-rider" likely referred to someone who worked on boats that transported goods, possibly a sailor or a merchant.
* A derogatory term: Given Smaug's arrogance, he might have taken "barrel-rider" as a term of derision, signifying a low-ranking, commoner person. This would have further fueled his contempt for the people of Lake-town.
* A lack of respect: The title would have indicated to Smaug that the speaker was not someone of importance. Smaug likely viewed himself as superior to the entire human race, so the fact that someone would refer to themselves as "barrel-rider" instead of their actual name would have been insulting.
It's important to remember that Smaug was a dragon, a creature known for its greed, arrogance, and disregard for human life. He was not known for his subtlety or understanding of human social structures. The title "barrel-rider" would likely have been interpreted through the lens of his own inflated ego and disdain for those he considered beneath him.