Early Primitive Elvish
qḷřḷ
root. QḶŘḶ
qḷðḷ
root. *quiet
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A root in the Qenya Lexicon of the 1910s given as ᴱ√QḶŘḶ [QḶÐḶ] “quiet” with derivatives like ᴱQ. qilde “quiet, rest, hush” and ᴱQ. qildi- “rest, stay quiet, be quiet” (QL/78); immediately below it had a variant form ᴱ√QILI² (QL/77). It had derivatives in the contemporaneous Gnomish Lexicon as well such as G. cwîl “quiet, peaceful, gentle” and G. cwildred “bat (animal)” (GL/28).
However, in a table of primitive forms elsewhere in the Gnomish Lexicon, primitive qḷd was deleted (GL/23) and in yet another section Tolkien gave a distinct base form gwil-² with derivatives like G. gwîl “cessation, quiescence, leaving off” and G. gwilthi “peace”, with a possible primitive form ᴱ√u̯il marked by Tolkien with a “?” (GL/45). Tolkien compared G. gwil-² to Q. qil-, so perhaps in this instance he imagined primitive initial gʷ > kʷ in Qenya, following the pattern whereby initial voiced stops were unvoiced.
Some form of this root seems to have survived in the Declension of Nouns from the early 1930s to serve as the basis for ᴹQ. qilir “quiet” < ᴹ✶ku̯ilẹz (PE21/34).
Neo-Eldarin: I think it is worth positing a Neo-Eldarin root ᴺ√KWILID “quiet” to preserve these earlier Quenya words.