Middle Primitive Elvish
slus
root. whisper
srus
root. whisper
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
A root in The Etymologies of the 1930s with variants ᴹ√SLUS and ᴹ√SRUS; it was glossed “whisper” with derivatives like ᴹQ. lussa- “to whisper” and ᴹQ. lusse/N. thloss/floss “whispering sound” (Ety/SLUS). The only sign of ᴹ√SRUS was the variant form N. thross of N. thloss/floss.