A forest in northern Dorthonion corrupted by Morgoth and turned to darkness (S/155). Its name is a compound of taur “forest”, nu “under” and fuin “night” (SA/taur, fuin). The final element was often translated “nightshade” (S/155, WJ/56), but this is an allusion to the other name of this forest: Deldúwath “Deadly Nightshade”.
Conceptual Development: This name dates back to the earliest Lost Tales, and always had the elements taur and fuin. Its development was G. Taurfuin “Forest of Night” (LT2/47) >> N. Taur-na-Fuin “Forest of Night, Deadly Nightshade” (LB/34, SM/26, LR/133) >> S. Taur-nu-Fuin “Forest under Night(shade)” (S/155), with the middle preposition changing from na “of” to nu “under”.
In some older writings, this forest’s name was translated “Mirkwood” (LR/282, WJ/239) and in at least one place Tolkien decided that Taur-nu-Fuin was the proper Elvish name of Mirkwood (UT/281). However, the canonical Elvish name of Mirkwood was Taur e-Ndaedelos “Forest of the Great Fear” (LotR/1134).
taur (“great wood, forest”) + nu (“under”) + fuin (“night, gloom, darkness”)