A “term of art” is a phrase that is used to describe something in a specific industry. For example, it is perfectly reasonable to describe something as being “abaft the beam” if you’re sailing but on land we just say “it’s a little further up”. Hairy arms is a design term of art that comes out of the Disney studios and it refers to adding something you know will get edited to call attention away from the rest of the design that you want to stay as it is. Listen to this piece on NPR to get a better idea. Very clever.