Akkadian

birku, burku - knee, lap (CAD B 255, AHw. 129)

-­u-­ in Akk. (alongside with ­-i-­) and Arm. (Syr. and Mnd.) is likely a secondary development under the influence of b-­. For the meaning ‘lap’, see *barak-­ ‘chest, thorax’. See discussion in [Holma 1911 95] where the author tends to treat birku ‘knee’ and birku ‘genitals’ as the same word, quoting, at the same time, Arb. rakab-­ ‘pubis’ as a possible cognate to the latter.

PS
*bi/ark- - knee (SED I No. 39)
Cf. *rVk(u)b(­at)­ ‘knee’ (No. 232) and discussion in [GD 163], where separation of these two roots is also favored.