In an appearance on last night's Late Show with David Letterman, the 25-year-old admitted she got her tattoo with two of her Mamma Mia! co-stars to commemorate their bond together.
The Red Riding Hood actress said: 'I got it in Los Angeles, it's a north American tattoo and I got it on my foot.
'I thought, I don't really like tattoos that much but I wanted to know what it felt like and one night at dinner it seemed like a good idea.
'We might have been having a beer or three, you know those British people, I tell you, they just can't stop.'
She added: 'It's pretty right, it's my handwriting…It's a British slang term, it's a term for a female body part.'