Tantra -- From Sanskrit tantram, which means "weave".
Tantra -- Type of Hindu religious book, 1799, from Skt. tantram, lit. "loom, warp," hence "groundwork, system, doctrine," from tan "to stretch, extend," from PIE base *ten- "to stretch, extend" (see tenet). Hence, tantric (1905), used loosely in the West to denote erotic spiritualism.
http://www.etymonline.com/index.php?term=tantra
https://en.wikipedia.org/wiki/List_of_English_words_of_Sanskrit_origin#cite_note-183
Gonda says about the suffic tra-
The Sanskrit words in –tra < Indo-European –tro>, when neuter, are generally speaking, names of instruments or sometimes names of the place where the process is performed. The former category may occasionally express also a faculty: Sanskrit śrotram "organ, act or faculty of hearing"; jñātram "the intellectual faculty"; or a "function": hotram "the function or office of a hotar priest". [Gonda : 250]
http://www.visiblemantra.org/etymology-of-mantra.html
