Loki didn't even remember what the argument was about or how it had started, but it didn't matter. He argued with his brother so often that he'd memorized an entire list of spells to cast when the arguments were over (and, he usually lost, because Thor would try and escalate it to a physical fight, which Loki wasn't confident he would win at.)
Loki knew how much his brother hated to be embarrassed in any way, which is why he knew this specific spell would work on him. However, Loki wanted to make sure it really worked, so, he knew he'd have to spend more time with his brother that evening. He wasn't sure if he was looking forward to it or not. He supposed it depended on how things ended.
It was Thor who suggested that the two drink together to make up, but, instead of turning up his nose and leaving, Loki agreed, and Thor was clearly overjoyed. The two ended up in one of the sitting rooms in the castle, each in their own armchair. While Loki didn't object to drinking, he usually avoided consuming enough alcohol that he couldn't think straight, and tonight was no exception. While Thor downed drink after drink, Loki sipped on the same glass for at least an hour. Thor commented on it, saying that Loki probably couldn't hold his alcohol, and Loki had to stop himself from grabbing another drink and downing it just to prove him wrong.
After about an hour and a half of drinking, Thor was decently intoxicated, and he was shifting in his seat a bit. This had been exactly what Loki had been waiting for, so he picked up on it immediately.
Before Thor could come to the realization that he needed to piss, Loki stood. "Let's go on a walk, Brother."
Thor seemed confused, as he should be, since Loki hardly wanted to be around him, but, drunk as he was, he went along with it. Loki led him out of the room. He worried a bit, since Thor was not entirely steady on his feet, and Loki didn't particularly want to catch him if he tripped. He also didn't want to explain to anyone why Thor had bashed his nose on the concrete from stumbling around drunk under his watch, though. So, he just hoped Thor wouldn't fall.
They'd barely gotten to the gardens before Thor spoke up about his need. "I gotta piss..." He mumbled, his words slurred.
"Well, we're already out, so you'll have to wait until we get back." Loki replied. Thor wasn't shy, and if he had this predicament while he was sober, Loki had no doubt that he'd just walk over to the nearest tree or bush and go. But, he seemed to make sense of Loki's logic, and he didn't protest.
Loki wouldn't admit that he enjoyed watching his brother squirm. No, in his mind, all the enjoyment he was getting was simply because his elder brother was sure to make a fool of himself, pissing his pants like a child. That was all he wanted. Nothing more.
Thor did squirm as they walked, and soon, Loki was catching his brother's hand move to his crotch, giving himself discreet squeezes. He did his best to keep up a conversation, asking Thor about his friends and whatever else he could think of to keep Thor's mind off asking to return inside so he could relieve himself. Thor was easily distracted, so this worked very well.
They'd done the entire loop of the garden, and were heading back inside when Thor suddenly doubled over, completely tensing up.
"Something wrong?" Loki asked innocently. "Ah, that's right, you needed the bathroom. Well, we're almost back, and-"
"Too far.." Thor groaned, looking around frantically. "I've gotta piss...now.."
"No." Loki sounded firm as he answered. "We're almost there. Are you really about to piss yourself right here?"
"I'm gonna-"
"Not out here, you're not. Look, the door is right up ahead. Come on, or must you relieve yourself here like a child?" Loki enjoyed this control. He usually didn't get it over Thor.
Thor did give in, though, and he was shuffling toward the door with Loki, his hand gripping his crotch. It was dark, and Loki wondered whether it was a shadow or a small wet patch that he saw on Thor's pants.
Now was the fun part. When they were just steps from the door, Loki mumbled another spell, and he heard the lock on the door click.
Thor, between his intoxication and desperation, noticed nothing, and grabbed the door handle, jiggling it before swearing under his breath.
"What? Open it.." Loki said.
"It's locked.." Thor pulled at the door handle. Loki was sure that if he was sober and not about to piss himself, Thor would've broken the door down. He banged on the door.
There was an audible hiss for a second or so, and then Thor grunted and it stopped. Loki was now sure that there was a wet patch, rather than a shadow, on Thor's pants. Loki made no attempt to help with the door, his eyes locked on his crumbling brother.
"Ah...no..." Thor soon said, clearly to himself. The hissing noise came back, for just a few seconds, before it stopped. It was not even silent for thirty seconds before the noise started again. The starting-and-stopping continued a few more times, with the hissing becoming longer, before soon it just didn't stop, and Thor was soaking himself. The man visibly relaxed, panting as his bladder emptied. Loki couldn't help but wonder how he held so much, despite the spell.
Before he was even done, Loki heard the lock on the door click, and then it swung open, with one of the palace servants immediately apologizing that they'd been locked out. She stopped mid-sentence, though, going bright red as she realized that she was witnessing the next king of Asgard wet himself like a child.
Loki smirked, and once Thor had finished, he grabbed his brother's arm. "Looks like you should go bathe, brother." He said.
"Yes...okay..." Thor allowed himself to be dragged inside, embarrassed despite how drunk he was.
Loki could only hope this information would spread around the castle. And, if not, he simply wouldn't reverse the spell.