Comments: The AK root is attested within the compound *pćǝħá-śħa 'evening' (Ad. pč́ǝħā-śħ, Kab. pśǝħa-śħa), where the first component by itself means probably 'dusk; sunset/sunrise' (see *bǝćʷǝ). Despite Shagirov (2, 27) the component śħa here means 'evening' by itself and has nothing to do with PAK *śħa 'head'.
The correspondence PAK *śħa = Ub. šʷǝwá is exact and points unambiguously to PWC *śʷǝ-Łʷa. This stem itself is probably an old compound, with probable external parallels for both parts).