Jewish Babylonian Aramaic

paḥdīn - testiculi (Jastrow 1151)

(pl.). A similar shift of meaning see in Akk.paḫallu ‘thigh’ vs. ‘penis’ in other Sem. (cf. *paḫiḏ- - hip, thigh No. 210) and in Akk. birku ‘knee; lap; male and female sexual parts’ [CAD b 255]. 

PWS
*paḫiḏ- - hip, thigh (SED I No. 211, Kogan 2011: 225)
On the semantic shift ‘hip, thigh’ > ‘tribal union’ see Kogan-Militarev 2003 295-296.