'Mousy' is a term used to describe boring, flat hair. Usually a light brown colour, with no 'wow factor'. These days most people with mousy hair will either have blonde streaks put in it to liven it up, or colour it a darker, more interesting brown. It doesn't have to be boring in a straight and flat sense, the hair can be frizzy, straight, curly, wavy etc etc... it just lacks... oomph.
Mousy refers to the hair color. It usually means that the color is drab and dull.
Messy hair, with lots of grease in it. So a greasy look, usually not attractive.
Hair that isn't shiny, boring, usually a light brown color.
looks likes its wet but you can style it any way
like messy hair.