ok so the other day my mate from school came for a night out but it was raingin and we didn;t fancy it so he said he wanted to play some poker- so I took him to play the £20 triple at gala and do some drinking

I was sat with a young gala reg I know and we were drinking and messing around and did some silly last longer prop bet and I put a £60 bounty on his head and made him leave the cash under his stack so everyone would be enticed by it. He ended up busting me and I tiltedly snatched the £60 bounty back, then after the table were bantering with me needling me one of the guys said to me
"if you really had balls you'd be your auntie" or something liek that, some such saying that involved balls and auntie, I smirked politley then I asked him wat it meant and he said I'd find out when I was older, then I asked again and everyone laughed lol
Has anyone heard this expression before, what is the actual saying and wtf does it mean?
feel pretty naive but Jamie/rob dont know and google is drawing a blank, I dont know where else to turn.