A ruffle is a pleated piece of decorative fabric often used as trim on clothes, like the ruffles on a pirate’s shirt. To ruffle is to make someone lose their composure. Argh.
The word ruffle is mysterious, but it might be from the Low German word ruffelen which means "to wrinkle." A ruffle on the bottom of a dress is like a fancy wrinkle. To ruffle someone is to upset them, and they’ll need to get straightened out. Often, this word is used in the expression "ruffle their feathers." Hearing bad news will ruffle most people's feathers. An unruffled person and an unruffled shirt will both be smoother.