Having a drab, pale brown hair color. Almost gray (ish) In easier words: A REALLY UGLY BORING HAIR COLOR WHICH NEEDS TO BE STYLED PROPERY (brown hair always falls under this definition . . . sometimes blonde, but mostly brown)
You know that your hair is mousy if it's not necessarily a medium brown shade, not necessarily a blonde, just in the middle... with a hint of gray.
Lina: "That girl has a really pretty face!"
Maria: "Yea, obviously. But look at her hair... it's like a tangly light brown gray split ended... style... it ruins her face! What's the name for hair like that?"
Lina: "You're right. It's called "mousy hair". Thank GAWD my hair is not like that.."
Maria: "That's only cause you dye it"