PS

*kp - to bend; to encircle (Kogan 2015: 443)

likely related is PWS *kb - to be curved

Akkadian
kapāpu - to bend, curve (CAD K 175, AHw. 442)
Akkadian
kippatu - loop, tendril (CAD K 397, AHw. 482)
Hebrew
kippā - arch, doorway, bow; skull-cap (Jastrow 635)
pB.
Hebrew
kpp - to bend, to bow down (HALOT 493)
Jewish Palestinian Aramaic
kyph - dome, vaulted chamber, soft round object (DJPA 256)
Jewish Palestinian Aramaic
kpp - to bend (DJPA 267)
Syriac
kpy - to bend, bow, curve (LSyr. 323, SL 642)
Mandaic
kup, kpa, kpp - to bow, bend, curve (MD 208)
Arabic
kff - to sew the edge; refrain (Lane 3001)
Arabic
kiffat- - bezel, collet (Lane 3001)