I did an online search at the Japanese National Library site for go and tesuji, and the earliest go book I found was New Style Igo Life and Death Research, vol. 1 ( http://dl.ndl.go.jp/info:ndljp/pid/927127 ) by Suzuki Tamejiro. The first part of the book is devoted to tesuji. It is plain that Suzuki expects his readers to be familiar with the term, so it should have been in common use for some time. Also, the search located a non-go book from 1904 that briefly talks about tesuji and shudan (technique) in go. So clearly the term was in use in the 19th century. It could have been used in go books that the search did not uncover. Suzuki's book shows up because tesuji is in the table of contents, I believe. But I do not remember seeing it in earlier books.
The first part is divided into three sections, death, life, and semeai. Within each section the examples or problems are placed under different headings. Problems 1 - 8 fall under the heading, 2-1 nakade, problems 9 - 16 fall under what we now call the monkey jump and similar shapes, problems 17 - 22 fall under other tesuji. From this we may infer that the 2-1 nakade and the monkey jump are tesuji. No surprise there.
The next heading is 4 point, 5 point, and 6 point nakade. A look at the problems under this heading shows that they are not about plunking a stone down on the vital point, but about building those dead shapes. The remaining headings in the death section are descent tesuji, other tesuji (again), and under the stones. The nakade and under the stones headings suggest, as John Fairbairn indicates, that tesuji need not be a single play.The examples also make clear that tesuji may not be best play.
Gotta run. I'll post some examples later.