What does Thoreau mean when he uses the phrase, in Walden, "...sensuality to imbrute?"
The quote you are referring to comes from page four of Walden and reads, in total, like this: "Any nobleness begins at once to refine a man's features, any meanness or sensuality to imbrute them."
In order to understand what Thoreau is trying to say we need to have a good idea of the terminology being used in the statement. The tricky word here is "imbrute." This word means "to make a brute out of something...to make something brutal." Keeping that in mind, it would appear that Thoreau is trying to say that when a person acts in kind, intelligent, thoughtful ways his physical features are refined. To refine, in this sense, would mean "to make more detailed or better." So, behave well as a noble person would and it will reflect in your face.
Contrast that with what happens if you are a mean person who is governed by his/her passions. According to Thoreau, these types of behaviors will also take a toll on your features by making you look more like a brute. Brute, here, would be used to mean "like an animal." So, behave badly and it will reflect in your face as well by making you look like a cruel sub-human.