Image source, Getty ImagesByMark SavageMusic correspondentIn 1980, Dolly Parton was filming her first Hollywood movie, 9 To 5, and between scenes she had an idea for the tune that would become the movie's theme song. But her guitar was banned from the set - so she made the beat by strumming her acrylic nails together.

The sound, she realised, was just like the typewriter used by her character, a married secretary who is constantly belittled and sexually harassed by her boss. Excited, she ran over and sang the song to her co-stars Lily Tomlin and Jane Fonda, who was also the film's producer."

Lily and I looked at each other and we had goosebumps," Fonda said. " And we knew, this is not just a movie song, this is an anthem."

Released in November 1980, 9 To 5 became Parton's only US number one as a solo artist, and earned her an Oscar nomination for best song. She loved to tell the fingernail story, but 9 To 5 has another legacy - for its powerful statements about working life and women's place in the corporate world.

You can probably sing the hook off the top of your head. " Working 9 To 5 / What a way to make a living!"

It's jaunty and catchy. The sentiment is universal.

Work sucks. But dig deeper and Dolly's expressing real fury."

It's a rich man's game, no matter what they call it / And you spend your life putting money in his wallet," she sings. " They just use your mind and they never give you credit."

The lyrics tapped into real frustrations and gave working women a voice. But there's a reason why you might not have noticed the anger that fuels Dolly's song.

She simply doesn't sing it that way - even though she had every right to, having navigated the sexism of the music industry from the age of 13. Figure caption, Dolly Parton: The life of an iconic country singerHer early breakthrough came in 1967 when she was hired as the "girl singer" on The Porter Wagoner Show, a TV showcase for country singer Porter Wagoner that reached millions of viewers each week.

Together, they released 21 hit duets including Just Someone I Used to Know, If Teardrops Were Pennies and Please Don't Stop Loving Me. But as Parton's star rose, she began to outshine her co-star.

Eventually, she realised she needed to strike out as a solo artist – writing I Will Always Love You as a letter of apology to her former mentor. He responded by suing her for breach of contract in 1979, demanding $3m (£2,2m).