Imitation of a dog obediently and enthusiastically acknowledging an order. Said in response to receiving a curt order. Delivered an in a loud and sharp manner.
a less trendy way of using 'LOL' to represent laughing out loud when chatting online. You can use capital letters to represent harder laughter, and place them in any sequence depending on the type of laugh . 'ruFF' would be used if at first you just smiled, but then realized that what you read (or typed) was indeed, pretty funny.
in this case, it wasn't terribly funny, but enough to get a smirk, then at the end, Tim realizes Colin is being especially silly and is forced to laugh.