The Proto-Semitic word rkb
Word rkb
Meaning knee

Comments
Kaye, Alan S. "Christopher Ehret. Reconstructing Proto-Afroasiatic (Proto-Afrasian): Vowels, Tone, Consonants, and Vocabulary", University of California 1995

Other words that might be of interest
*yad / ʔid ‘hand’, θ̣pr ‘nail, claw’, ɬ(a)ʕp ‘foot’, rgl ‘foot’, ɬp(t) ‘lip’, pay- ‘mouth’, wsm ‘to sleep’, d(a)nb ‘tail’, dam ‘blood’, tady ‘breast’, ṯ̣upr ‘fingernail’, ʾudn ‘ear’, bayṣ̂ ‘egg’, ʾam(m) ‘elbow’, ʿayn ‘eye’, ṣibʿ ‘finger’, šāḳ ‘leg’, ŝaʿr ‘hair’, kapp ‘hand’, rāḥ, rīḥ ‘hand, palm’, libab ‘heart’, ḳarn ‘horn’, ŝap ‘lip’, ʾanp ‘nose’, naḫīr ‘nose, nostrils’, ʾadam, ʾadīm ‘skin’, šin ‘sleep’, lišān ‘tongue’, šinn ‘tooth’, kanap ‘wing’