I'm not a native speaker, but it seems to be used in a sentence like "Unataka kulala, lakini wapi" – "You want to sleep, but you can't." Or "Nilijaribu kuongea naye, lakini wapi" – "I tried to reason with him, but to no avail."
Google Translate gives 'but where?'. Might this make sense in e.g. 'Ninataka kulala, lakini wapi?'? I.e. I want to sleep, but there is nowhere that I can't.
I will ask my East African friend next time I see him.