Baptist minister J. M. Pendleton wrote a series of articles which appeared in The Tennessee Baptist, a periodical whose editor was J. R. Graves. These articles dealt with the "recognition" of Pedobaptist (infant-baptising) ministers, and were later published by Graves as a pamphlet titled An Old Landmark Re-Set. Pendleton and Graves agreed that Pedobaptist ministers should not be allowed to preach from Baptist pulpits. Evidently, Southern Baptists who disagreed with Graves and Pendleton on the issue of Pedobaptist ministers, took the initiative in labeling those with such convictions "Old Landmarkers". Even if so, Graves had no problem adopting and using it -- and possibly relished it. Read, for example,Graves' preface
to Old Landmarkism, What is it?, a book he wrote in 1880.
From Cathcart's Baptist Encyclopedia: "The origin of the term old-landmarkism was as follows: about the year 1850, Rev. J. R. Graves, editor of the Tennessee Baptist, published at Nashville, Tenn., began to advocate the position that Baptists cannot consistently recognize Pedobaptist preachers as gospel ministers. For several years he found but few to sympathize with this view. Among the few was Rev. J. M. Pendleton, then of Bowling Green, Ky., who in 1854 was requested by Mr. Graves to write an essay on this question, 'Ought Baptists to recognize Pedobaptist preachers as gospel ministers?' The essay was published in four consecutive numbers of the aforesaid paper, and afterwards in the form of a tract. The title given to it by Mr. Graves was 'An Old Landmark Reset.' The title was considered appropriate, because there had been a time when ministerial recognition and exchange of pulpits between Baptists and Pedobaptists were unknown. This was an old landmark, but in the course of years it had fallen. When it was raised again it was called "an old landmark reset.' Hence the term 'old-landmarkism,' and of late years, by way of abridgment, 'landmarkism'."