This a compilation answer of all the comments above with a few of my own thoughts.
First, The Hobbit states clearly in Chapter 1:
Instead, it was a very old-looking Dwarf on the step with a white beard and a scarlet hood... And "Balin at your service!" he said with a hand on his breast.
Balin, while technically being younger than Thorin, Balin ...
Tolkien was a linguist. He studied languages and even invented them for his books. Languages fascinated him. He was a huge nerd about languages. You know what nerds do when they're really into something?
They take every opportunity to be a little shit about it.
That's what Tolkien was doing here. He saw an expression he found odd, thought about all the ways ...