One of the Primitive Adûnaic roots Tolkien used to illustrate the processes of Primitive Adûnaic word formation (SD/422-5). It also seems to be the root of words such as kulbu and kulub “root (as a kind of plant)” (SD/431), perhaps an deliberate pun by Tolkien.
Primitive adûnaic
-k
suffix. *masculine suffix
kulub
root. *root (as a kind of plant)
A (primitive?) masculine suffix used in the formation of (male) mîk “baby” from the root ✶Ad. √MIYI (SD/427). Tolkien stated that that k was one of the “favoured” masculine consonants (SD/427).