I think it's probably a bit old-fashioned to use for girls growing up today, but for grown women who were children 20-30-40 years ago I've just assumed it reflects the way they felt about themselves at the time - that they behaved or acted in a more "boyish" way than was expected of girls at that time or in their particular social setting.
Even though the tomboy concept is outdated you still find IMO that activities that require courage, persistence and physical effort are considered masculine (and have higher status) than activities that require sensitivity, empathy, attention to detail, that are considered feminine and have lower status. People are impressed that my dh can cook, clean and remember our son's shoe size - but they're a lot more impressed when I do outdoorsy stuff. It's pretty unfair.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett