Pioneers were originally known as colporteurs.
*** yb75 p. 61 Part 1—United States of America ***
For many years prior to 1914 zealous colporteurs—Christian men and women “aglow with the spirit”—had been spreading the good news far and wide. (Rom. 12:11) The colporteur service began in 1881, when Zion’s Watch Tower carried the article “Wanted 1,000 Preachers.” To persons without dependent families and who could give one half or more of their time to the Lord’s work a plan was suggested. It was that they go into large and small cities as colporteurs or evangelists. For what purpose? Said the Watch Tower: “Seek to find in every place the earnest Christians . . . to these seek to make known the riches of Our Father’s grace, and the beauties of His word.” Bible publications were to be placed in the hands of such persons, and colporteurs were permitted to pay their own expenses with money received from literature placements and Watch Tower subscriptions that they obtained.
*** yb79 p. 128 Canada ***
Colporteurs were well organized by the early 1930’s... About that time, colporteurs began to be called pioneers.
Definition for colporteur:
More info » Source - Dictionary.com - Answers.com - Merriam-Webster - The Free Dictionary The NWT does use the word PUBLISH(ING) which the WTS uses to describe jws, even pioneers.
(Acts 13:4-5) 4 Accordingly these men, sent out by the holy spirit, went down to Se?leu´cia, and from there they sailed away to Cy´prus. 5 And when they got to be in Sal´a?mis they began publishing the word of God in the synagogues of the Jews. They had John also as an attendant.
(Acts 15:36) . . .Now after some days Paul said to Bar´na?bas: "Above all things, let us return and visit the brothers in every one of the cities in which we published the word of Jehovah to see how they are."
(8 more times in Acts)
No "pioneer" who were called "colporteurs" earlier.
The term appeared fairly frequently in 1920s Watchtowers when Bible Students signed letters describing themselves as "pioneer colporteurs". Watchtowers described the apostle Paul as a pioneer (by preaching in virgin territory) and by the looks of a September 1931 WT article, the term referred similarly to fulltime preachers who travelled, often pitching tents to sleep. The term appears liberally in 1932 Watchtowers, again referring to fulltime travelling preachers.
By late 1932 the term seemed to have replaced "colporteurs" (the September 1932 WT reads: "This “crown of glory” is not bestowed because one has been elected to an office by the votes of men, but is bestowed upon all the faithful, including the faithful pioneers, whom we sometimes call “colporteurs”, and all others who are faithful and are made members of the ‘elect servant’.
A Nov 15 1933 WT article refers to the number of "pioneers and auxiliaries" at work in the US. The term seems to have been broadened in later years to include anyone plodding around neighbours, wasting a set amount of time to impress others with their level of dedication. Oh, and spirituality of course.