Some beetles have bushy protruding antennae, so the expression “beetle-browed” came to describe someone with prominent, bushy eyebrows. This was often used in a pejorative fashion, perhaps because someone’s eyebrows become more pronounced when they are scowling or frowning. Here, Shakespeare coins the verb “beetle o’er” to mean overhang in a threatening manner (as eyebrows would overhang the eyes of an angry or upset person).