The Saturdays singer — who tied the knot with the footballer in 2014 — spilled the beans during a campfire chat with her fellow contestants on the ITV reality show.
She told the others that she decided on the unusual gift because she didn’t know what else to buy for a man who had “everything”.
“I got Wayne a cast of my a***,” she said.
“He used to say it as a joke. Back when he was playing football. What do you buy someone who has everything? He always used to be like, ‘I just want a cast of your a***’ so one year I was like, ‘Fine, that’s what you’re getting.’
“So I took my two best mates and you literally have to bend over and this guy literally has to slap all of this stuff on. I was literally having to bend over like this (mimics bending over a table) while he did it. It’s like papier-mâché... slapping it all on.”
The singer, who has two sons with Wayne, said the cast is hanging on a wall upstairs in their house and that she sometimes forgets it is there.
“I remember I was doing a Zoom or something and everyone online was like, ‘Is that your bum in the background?’” she said.
“And I was like ‘Ummm…’.”
Wednesday night’s instalment of the reality show saw Frankie and the other campers returning to Gwrych Castle after they were removed because of Storm Arwen.
The campers all said they were happy to be back after their break.
TV star Louise Minchin said: “It’s not quite like being at home but it is like coming home and it’s absolutely wonderful. I’ve missed everyone!”
Speaking in the Telegraph, Arlene Phillips said: “It was strange coming into camp. Weirdly it just really felt like no big deal, I’m just coming home to finish off something I’ve started.”
I’m A Celebrity... Get Me Out Of Here! is on ITV and ITV Hub.
Watch: Who is I'm A Celeb's Frankie Bridge?