Legend said that Soulmarks were in truth a curse laid on the Nine Realms by Odin. As the tale went, the Allfather was tied up in a dispute between two families who demanded that the secret marriage of their heirs that had been discovered a week before, be annulled. They had far more suitable marriages arranged for them and this would simply not do, they said and hurled insults long and hard at the other hoping to clear their name by tainting the others'. So long was this trial that the Allfather missed the birth of his first child, a son.
Frustrated and beyond annoyed by their petty feud, the Allfather wove an enchantment upon them as punishment. Their families would now and forever have the first words spoken to them by their One painted into their skin.
But he was enraged and in his rage, his power grew so great that it extended far beyond the walls of the palace, beyond the spaces between Realms, beyond Death itself and embedded itself into Everything.
Or so legend said. And as was often the case, legend was wrong.
It was actually Odin's father, Bor, who had done so but the rest of the tale was quite accurate. Anyone who bothered to look for the tale itself instead of listening to what everyone else said knew the truth. Pity then, that there were so few on Asgard who cared to know the truth. Loki was one of the few.
He had always thought Soulmarks were an oddity. So arbitrary were the rules, some got theirs when their One was born, some when the crossing of their paths had been set into motion some even got them after they had met only to realise in hindsight that their One had been in front of them the whole time.
People, as Loki had found out the hard way, didn't like being told what to do. They didn't like being told whom they were predestined for and thus Soulmarks meant truly very little. Men and women still fell in love with 'the wrong'uns' and lamented it with mead when it ended. Brothels still operated in full swing even when the flesh revealed would disclose whom they were really meant for.
Life moved on.
Loki Odinson never really expected to get his Soulmark at all, let alone to get it in the middle of lessons. He had just finished his workbook and was busy staring out the window while Thor got scolded for his terrible penmanship when a cold burn began to bloom on his shoulder.
The boy, as he had been then, was surprised but hid it well, deciding to save it for his eyes. After all, Thor hadn't gotten his yet and his brother always sulked for weeks when Loki surpassed him in anything. He wouldn't be surprised if Thor was mortally offended by this even though Loki didn't have anything to do with it at all.
'But I'm the older one!' Thor would say and stomp his foot before refusing to talk for Loki and that just would not do.
He had a prank in mind that called for Thor's brute strength.
So he kept quiet and took to hiding it. From then on, his tunics bore high collars and sleeves even on the warmest of summers and he never took it off even when they went swimming in the creek. Secrets were something Loki adored and he would keep this as long as he could.
It wasn't a particularly nice Soulmark anyway. Why would his One possibly say "This does not bode well", upon meeting him?