1. The hair atop a steady girlfriends head usually without style or care for how it may look. Sometimes one single length, straight and with a side bang as a way of holding on to any sort of femininity that may still be there. Also may be in a constant ponytail state. A definite sign of the girlfriend no longer caring about how she looks because she obviously thinks her boyfriend will love her no matter how she may look.
2. A young woman's pubic hair which has not been shaved for a duration of time. Again thinking because she is in a steady relationship, she no longer needs to keep it short and clean any longer and can now let it get to the length of a yetti. Also a strategy used for preventing sex and establishing many nights of cuddling instead.
Girl 1: Steve broke up with me over 3 weeks ago and I still have girlfriend hair!
Girl 2: You had better get a cut and style on the quick because you're starting to look pathetic.
Girl 1: Last night I saw Angela change from her jeans into her sweat pants. She's totally got girlfriend hair going on, so much so in fact that it's causing a slight bulge in her panties. Nick must be angry at all the sex he's not getting. She better take a trimmer to that rat's nest before he drops her.