The first idea is that of ‘winnowing’ or sifting, from the ur-Bantu stem -ĸuŋga, meaning ‘sift’. And the second centres on what I would argue is the nominalised form of the first stem, -ĸuŋgu – which means ‘bitterness’ or ‘poison’.
Colloquial usage tells any speaker of isiZulu that when you say ‘kubuhlungu’, you mean ‘it’s sore’. It’s an essential phrase for any child growing up in a bilingual family. But what are you actually saying? ‘it is bitter, as though I have been struck by a poison arrow’.
So how does ‘bitterness / poison’ relate to ‘sifting’ things?
Perhaps the idea lies in the fact that some processing needs to occur before the poison can be useful – some sort of sifting in the form of concoction or distillation, to separate the good from the bad, the poisonous from the curative. It could equally lie in the fact that knowing poison from good herbs (and foodstuffs) could be difficult – especially when gathering from wild places, as is done when gathering umfino (‘wild spinach’) – and thus involved ‘sifting out’ the dangerous herbs from those that were edible.
Because there are many botanical, organic or chemical things that derive from this one stem – of the 25 associated words or stems, 16 are words relating to medicine, botany or chemistry. And these are words that are derived from either stem.
In terms of medical application, the terms derived from both stems focus on two aspects – poison (and poisonous concoctions, venom, shrubs etc.) and antidote for those poisons. Much like the Greek term pharmaka (poison/antidote).
But there is still the matter of bitterness, and of pain. That they are like poison to the body, needing to be balanced by other things.