The supermodel posed nude for the painter - who died last year aged 88 - and she has now revealed to Vanity Fair magazine he etched two swallows on the base of her spine, which could now be worth millions
Kate revealed the painter had told her he had learnt to draw tattoos using permanent ink and a scalpel when he worked in the Merchant Navy during World War Two.
The 38-year-old model said: "I said I liked birds and he replied, 'I've done birds. I've done it in my book.' And he pointed down at a painting of a chicken upside down in a bucket. And I said, 'No, I'm not having that.' And then he said, 'Maybe I should just do you.' And I thought, I'm not going to have a girl on my a***.
"So we decided on a flock of birds. It's an original Freud. I wonder how much a collector would pay for that? A few million?
"If it all goes horribly wrong I could get a skin graft and sell it!"
Freud painted a nude protrait of Kate when she was pregnant with daughter Lila Grace in 2002, which went on to sell for £3.9million at auction in 2005.
Kate revealed how she became friends with the artist because he invited her to dinner after naming him as the person she'd most like to meet in an interview.
She said: "I went to the house and he started (painting) that night. Couldn't say no to Lucian. Very persuasive."