A prefix appearing in Definitive Linguistic Notes (DLN) from 1959, meaning “hard, difficult” based on the root √GUR of similar meaning (PE17/154). Tolkien gave as examples of its use: urucarin “made with difficulty” and urcarne or urucárima “hard to make / do”.
Conceptual Development: On a rejected draft page Tolkien also had an unglossed prefixal form ur- apparently derived from √GUR “hard, stiff, difficult, cumbrous, slow”, with an unglossed example urunóte or urnótima, likely “✱difficult to count” (PE17/172). On this same rejected page Tolkien had ur(u) based on √UTHU “unsuitable, bad, improper, useless, wrong”, but since this derivation doesn’t make sense this is probably a slip or misreading for the prefix us-; see that entry for discussion.
Neo-Quenya: I believe that ur(u)- has the connotation “difficult, arduous” due to the inherent complexity of the task, as opposed to hrai- meaning “difficult, hard” due to resistance from the thing or people being worked upon.
ur(u)- prefix denoting difficulty (PE17:154, 172), cf. urcárima, urucarin