To look sharp is to look nice, classy. I think that's what you're referring to. If your friend shows up one day in a nice suit and tie, you might say "Wow, Eric. You're looking sharp today." But "look sharp" can also mean to act or move fast, to hurry up. "Look sharp, boys! They're headed right for us!"I don't know the reasoning behind that one, though. It's just one of those things, haha.