At first John didn't think about it.
"I'm making tea. Want any, Sherl?" John asked as he walked by clad in pajama bottoms and not much else. He ruffled the sleepy detective's hair affectionately as he passed by on his way into the kitchen.
"...What?"
"Tea. You want some?" John asked again. It was just like Sherlock to be so caught up in his thoughts that he couldn't hear a word anyone said.
"No I meant...what did you just call me?"
"Sherl. It's a nickname. Why, not a fan?" John shrugged, filling the kettle with water.
"..." Sherlock pulled his knees up against his chest, pondering the nickname. John smiled, it was during times like this that his boyfriend looked the most like a little kid. Whenever he was confronted with the problems of social interaction.
"I could always call you Sherly." John smiled, clambering onto the couch and pulling Sherlock down onto his lap. Sherlock squirmed his way into John's arms and lay his head against the doctor's chest.
"That's what my parents and brother used to call me. I rather not."
"Hm...what about...Lock?"
"What are you supposed to be, Key?"
John chuckled and pinched the detective's cheeks lightly, pressing a kiss into his curly dark hair.
"Well I like Sherl." He nodded. Sherlock made no protest against the name which John took as an okay to use it.
He didn't think about nicknames again until later that day.
"So anyway, three bodies all with the hearts cut out, their times of death only two or four minutes apart but miles away from each other." Lestrade sighed. "Serial killer for sure, but how is he moving so fast? Could be a cult."
"As always, Lestrade, you fail to see the obvious." Sherlock smirked, examining the bloody body with his usual macabre curiosity.
"Obvious to him. Of course." Lestrade commented just quiet enough that only John could hear.
"I'll need photographs of the other victims. Yesterday would be a good time." Sherlock commented, standing suddenly and walking back to John's side. As he did the doctor noticed a slight stumble in his step. Sure enough when Sherlock passed John he leaned ever so slightly against the shorter man for support.
"Sherl, have you been skipping meals again?" John sighed with concern. "What about sleeping? I know you haven't been to bed in awhile, I keep finding you on the couch."
Sherlock's head snapped up at the mention of his new pet name, and his pale face flushed with red.
"That is unimportant." He growled, attempting to flee the scene. John grabbed the man's wrist, holding him within scolding distance.
"Being on a case is no reason to not take care of yourself, Sherl, and being embarrassed doesn't mean you can walk away from me." John was stern faced on the outside but on the inside he was laughing at his discovery. It seemed that if he wanted Sherlock to behave all he had to do was call him by his new nickname. It was a new weak spot in an otherwise invulnerable man.
Sherlock spared a glance at Lestrade, who had a hand over his mouth to avoid laughing. Then Sherlock turned his glare on John.
"Oh no, none of that. Lestrade can send those pictures to us, we're going home and you're eating and taking a nap. Hear me...Sherl?" The corners of John's mouth jerked up into a smile.
Sherlock pouted, and his eyes were furious.
"...Agreed..." He huffed, retreating to go call a cab.
John turned at the sound of Lestrade exploding into laughter, and he couldn't help but laugh too.
"Wow, you two." Lestrade chuckled. "You two are really cute, huh?"