The phrase "dirty fingernails" usually refers to someone who works hard and gets their hands dirty doing manual labor or physical work. It can also imply that someone is not afraid of hard work or getting involved in difficult tasks. Full definition
All summer we would go camping where we rode our bikes everywhere & I would come home with dirty fingernails -LCB- I'm cringing at the memory -RCB- and my tomboy self thought it was super cool to wear a daily uniform of Adidas indoor soccer shoes, wind pants and a Patriots jersey. (sequinsarethenewblack.com)
As someone that doesn't love wearing crazy nail polishes but also doesn't love the look of dirty fingernails, this polish is the perfect medium. (allure.com)