An adjective glossed “hard, difficult” in Definitive Linguistic Notes (DLN) from 1959 derived from the root {√DIR >>} √DER of similar meaning (PE17/154). The primitive form was {✶dīrā >>} ✶dērā where [after the change to the root], the ancient long [ē] became [ī] as usual in Sindarin. This word clashes with dîr “man”, but can be distinguished because (a) it is an adjective rather than a noun and (b) its mutations are different, because dîr “man” is from ancient nd- while dîr “difficult” is from ancient d-, as in i nîr dhîr “the difficult man, (lit.) the man difficult”.

Tolkien gave one example of this prefix’s use in dirbedui “hard to utter, difficult to pronounce”. In its prefixal use, Tolkien glossed it as “tough (for lesser efforts)”, contrasting it with gor- “difficult (of things very painful and horrible to do)”; see that entry for discussion.

Neo-Sindarin: I believe that the adjective dîr has the connotation “difficult due to the complexity of the task”. It can be contrasted with rhanc, which I use to mean “difficult due to some resistance or obstacle” for purposes of Neo-Sindarin; compare Q. hranga- “thwart”.

Conceptual Development: The Gnomish Lexicon of the 1910s had similar-seeming G. diriol “tedious”, but that was based G. dîr “long (of time)” so was unlikely to be related conceptually (GL/30). In Gnomish Lexicon Slips modifying that document Tolkien considered the form duiriol “tedious” before reverting back to diriol (PE13/112), and there was no sign of this word thereafter except for its possible reemergence as dîr “difficult”.