It sounds almost slanderous to accuse someone as manly as Ernest Hemingway to have ever worn women's clothing, but it's true -even if it wasn't actually his choice. You see, Ernest's mother always wanted to have twins, so when Ernest was born only 18 months after his older sister, mummy dearest decided that was close enough:
Submitting to her twin fantasies, she started dressing Ernest up in Marcelline’s old clothes, despite the fact that they were little girl’s clothes—lacey white dresses with pink bows and the like. Soon his mom was buying two of everything and dressing her children in identical pink gowns and flowered hats. She would refer to the kids as her “sweet Dutch dollies” and actually tell strangers that they were her twin girls. To perpetuate the twin fantasy, Grace even held Marcelline back a year in school so that she and Ernest would be in the same grade together. Oh, and his mother so treasured the fantasy of Ernest being a little girl that she sometimes referred to him as “Ernestine.”
If you've ever wondered why the author placed such an importance on masculinity, hopefully that bit of trivia will help answer your question.