1. To experience a day when one's hair is unruly and looks unkempt. I'm wearing a hat because I'm having a bad hair day, OK? Talk about having a bad hair day—what am I going to do with this unruly mop?

2. To have a bad day. This phrase can also be applied to machines. I'd avoid the boss if I were you—he must be having a bad hair day because he's screaming at everyone he sees. I'm having a real bad hair day—everything has gone wrong. Do you think the coffee maker is really broken or just having a bad hair day?