The 22-year-old singer zei that he picked up the nickname after his fellow group members saw him naked after a concert.
Merrygold told the Daily Star: "We normally jump straight into the showers after a show. I jumped out naked and was waving my bum at the lads, messing about.
"They've nicknamed me 'Skinny Bum' ever since. They won't stop calling me it."
He added: "We call Oritse 'Hancock' because of his superhero-sized willy, Marvin 'Incredible Hulk' because of his big muscles and JB 'Bananaman' because [his group colour is] yellow."
