Prudence, as St. Thomas says, is "right reason applied to action." Shrewdness is a part of this.
Now a right estimate or opinion is acquired in two ways, both in practical and in speculative matters, first by discovering it oneself, secondly by learning it from others. Now just as docility consists in a man being well disposed to acquire a right opinion from another man, so shrewdness is an apt disposition to acquire a right estimate by oneself (ST II-II Q49 A4; emphasis added).He goes on to say, quoting Aristotle, "Shrewdness is a habit whereby congruities are discovered rapidly."
There are, perhaps, negative connotations associated with the word today, but it seems that Aquinas has none of them in mind. As a part of prudence, it is associated with virtue. It's a good thing to be inclined to acquiring right opinions on our own - not that there is anything wrong with being teachable, either.
Somehow this illustration comes to mind: you don't have to get down in a hog wallow to know that it's dirty. Perhaps a shrewd man - having never seen a hog wallow - demonstrates his shrewdness by figuring out on his own that it's dirty the first time that he sees it; whereas the merely docile (i.e., teachable) man might need to be thrown in :-)