Aeron, who doesn't seem easily rattled, seems to be absolutely terrified of Euron, and his thoughts about him, back before he was born again, involve "a rusty hinge" opening.
Stuff like this (From
Feast):
"That man is dead. Aeron had drowned and been reborn from the sea, the god's own prophet. No mortal man could frighten him, no more than the darkness could... nor memories the bones of the soul.
The sound of a door being opened, the scream of a rusty hinge. Euron Crow's Eye has come again. It did not matter. He was the Damphair priest, beloved of the god."
"The sound came softly, the scream of a rusty hinge. "Urri," he muttered, and woke fearful.
There is no hinge here, no door, no Urri."
"He had run before the Crow's Eye as if he were still the weak thing he had been, but when the waves broke over his head they reminded once more that man was dead.
I was reborn from the sea, a harder man and stronger. No mortal could frighten him, no more than darkness could, nor the bones of his soul, the grey and grisly bones of his soul.
The sound of a door opening, the scream of a rusted iron hinge."
"Even a can priest doubt. Even a prophet may know terror. Aeron Damphair reached within himself for his god and discovered only silence. As a thousand voices chanted his brother's name, all he could hear was the scream of a rusty hinge."
I also recall himself thinking himself feminine or a girl back then, (don't remember exact words, but in Victarion chapters men being raped are "used as women"). Basically it sounds like Euron would come to Aeron's and Urri's room and do something to them when they were young. Sexual abuse would fit, but it could of course have something to do with the death of Urri, or perhaps Euron dabbled in magic even back then, which I reckon would unsettle Aeron. But considering Finger Dancing is common with the Ironborn, I don't think simple violence could explain Aeron's attitude towards Euron. I'm going with rape until proven otherwise.