√NID root. “force, press(ure), thrust; will”
This root first appeared as ᴹ√NID “lean against” in The Etymologies of the 1930s with derivatives like ᴹQ. nirwa/N. nedhw “bolster, cushion” (Ety/NID). This sense may have replaced the deleted entry ᴹ√NID² “hurt, injure” with various words of similar form but different meaning (Ety/NID²). In Late Notes on Verb Structure (LVS) from the late 1960s, Tolkien gave a new meaning to this root as follows:
The basic stem NID- ... meant press, thrust, force (in a given direction) and though applicable to the pressure of a person on others, by mind and “will” as well as by physical strength, could also be used of physical pressures exerted by inanimates ... In T. and S. NID became used exclusively of “rational” purpose and will (PE22/165).
In this sense its derivatives included Q. indo “mind in its purposing faculty, the will”, whose Sindarin equivalent S. inn meant “some particular purpose or intention of an individual”. In Quenya the verb form Q. nir- retained the general meaning of “urge, press, force” for both physical and mental pressure, but in Sindarin the verb forms nídha- and nidh- both had to do with intent, the former very strong in meaning (“I am determined to do it”) but the latter somewhat weaker (“I will do it”).
A different precursor to √NID “press, thrust, force” appeared in a list of roots composed in 1959-60: √NIR “will, intention, conscious resolve to move or do” with derivatives Q. níra “will” and Q. nirmë “act of will”. The first of these words could also be a derivative of √NID, since intervocalic [d] usually became [r] in Quenya. Thus it seems Tolkien decided that Q. nir- was associated with “will” at least a decade before he wrote the note on √NID “press, thrust, force”.
Neo-Eldarin: For purposes of Neo-Eldarin, I would stick with the meaning of the root described in LVS from the 1960s, but the words for “cushion” might be salvaged in the sense of a thing that resists pressure. The derivatives of √NIR may likewise be based on √NID.
References ✧ PE22/165; VT41/13, 17