From the horse jockey to the disc jockey

Solution 1:

1. Jockey is the diminutive or pet-form of Jock, originally Scots and northern English. Jock is the Scotch equivalent of Jack.

OED, Oxford Dictionary of Word Origins (by Julia Cresswel), Origins: A Short Etymological Dictionary of Modern English (by Eric Partridge) and A Dictionary of Scottish Phrase and Fable (by Ian Crofton), all give the same etymology.

Hence, the origin of the word is also the original meaning and OED defines as below:

A diminutive or familiar by-form of the name Jock or John, usually with the sense ‘little Jock, Jacky, Johnny’; hence, applicable (contemptuously) to any man of the common people (chiefly Sc.); also, a lad; an understrapper.

OED's earliest citation is from a1529:

Kynge Iamy, Iemmy, Iocky my io.

_{J. Skelton Against Scottes (1843) 90}

A Dictionary of Scottish Phrase and Fable (by Ian Crofton) adds that the word Jockie was originally (in the 16th century) applied to stable lads, before coming to denote a professional rider. In the 17th and 18th centuries it was also used in English broadside ballads as a name for a Scottish soldier.

2. The informal meaning of jockey, (someone whose work involves the use of a particular object or machine) in expressions like computer jockey, desk jockey or disc jockey is an analogy to a horse jockey. It doesn't directly derive from the original meaning of jockey.

OED's earliest citation for this meaning is about driving a car and it is from 1912:

Some are, so to speak, ‘gentlemen jockeys’, and own, enter, and drive their own cars for the fun of the thing.

Collier's 28 Sept. 11/2

Additionally, the slang meaning of jockey is a driver as mentioned in a 1936 reference below:

Here is a short list of busmen's slang phrases:..Jockey (Driver).

Daily Herald 5 Aug. 8/4

3. Disc jockey was not the first expression where the term "jockey" was used to refer to someone outside the horse racing contexts.

The 1912 citation from OED has "gentlemen jockeys" that refers to the car drivers but it is not a set phrase.

Then, OED gives the following slang terms from The American thesaurus of slang (by Lester V. Berrey and Melvin Van Den Bark) from 1942:

• Automobile racer, auto or buzzer jockey,..speed jockey,..suicide jockey.
• Motorcycle racer, broadsider, jockey,..motor jockey.
• Commercial driver (bus, taxicab, truck),..jockey, motor jockey.
• Truck driver, truck jockey or spinner... Spec. juice jockey, a gasoline-truck driver; grunt-and-squeal jockey, a stock hauler;..suicide jockey, a nitro-glycerine hauler.

OED's earliest citation for disc jockey is from 1941:

Disc jockey solves vacation. Turning a program over to the public while the emcee is vacationing is big stuff from a listener's angle, WEBR is finding.

Variety 23 July 34/4

OED says that there is no evidence for the 1935 reference:

It has sometimes been suggested that the U.S. journalist and radio commentator Walter Winchell first used the term in 1935, but there does not appear to be any evidence for this.

OED also mentions the term record jockey (defined as n. U.S. colloq. temporary = disc jockey) which is from 1940 (slightly earlier than disc jockey):

[Quoting J. Kapp] The name bands are come on for the record jockeys who ride herd over not only Decca records but all the others.

Variety 3 Apr. 39/3