Who gave Scotty the nickname "Scotty"?

Question

On screen, the first time we hear the nickname "Scotty" for Montgomery Scott is in the episode "The Naked Time":

KIRK: I'll hold you to that half second, Scotty.

The reboot films go out of their way to create a foundational conversation where the nickname "Bones" for McCoy is used for what is clearly the first time; however, they give no such treatment to "Scotty". The nickname comes up all of a sudden in the 2009 film, several scenes after meeting Scott:

KIRK: How are we, Scotty?

SCOTT: Unbelievably, sir, the ship is in position.

To be clear, I'm not wondering why he has this nickname — it's clearly an affectionate reference to his family name and heritage. I'm wondering who gave it to him. Was it Kirk? Was it someone else in the crew, like McCoy? Or was it a nickname he was already fond of, and then encouraged Kirk and others to use?

The Memory Alpha article on Scotty simply says that he is "referred to as Scotty by his shipmates".

The official StarTrek.com database entry on him only says that he is "known as 'Scotty' as well as 'the miracle worker' to his longtime comrades James T. Kirk and crew".

Who gave Scotty his nickname?

Answer 1

James Doohan

According to Scotty's Memory Alpha page, it was James Doohan's idea to name the character Scotty:

(see 3:26 in the above video)

_{This is a bit of a tongue-in-cheek answer; I know that the OP is looking for an in-universe answer, but I thought I'dd add in this detail}

Answer 2

In the reboot universe, the tie-in novelisation for Star Trek suggests that Kirk was the first on the Enterprise to employ this particular nickname. That being said, Mr Scott has almost certainly heard it before, what with being Scottish and called Scott.

The engineer considered. Then he broke out in a wide, wild grin. “At the hearin’ about the dog they said that unless I straightened up I was going to the dogs. Aye, Mister Pointy-ear, let’s do it! What’s the worst that can happen? That I spread meself all over a wide corner of the cosmos? Better to go out in a flash than a footnote.” He looked over at the younger officer. “And you, Lieutenant—Kirk, was it?”

Kirk nodded. “I don’t have any choice, Mister Scott—Scotty.” The engineer didn’t chastise him for employing the nickname. “There’s far more at stake here than you yet realize. And I can’t do anything about it if I’m stuck here on this planet.” He smiled thinly. “No matter how convivial the company or engaging the surroundings.”