We are apparently expecting really freaking cold weather in the next few days. In the nearly 2 years (!!) that I've lived in Binghamton, NY, I don't think I've ever noticed the forecast specifically calling for "bitter cold" weather. Is that a technical term??? Apparently not, since it's not in my handy-dandy weather glossary.
However, a glance at an ordinary dictionary informs me that, in the weather context, "bitter" can mean "causing pain; piercing; stinging: a bitter chill."
Note to self: "bitter cold" is code for "weather that causes you physical pain." SWEET.