I get asked a lot about this. I will make it simple and to the best of my history knowledge.
First I will introduce you to one of the 12+ dialects of Swahili language....KINGWANA....Kigwana dialect is the Swahili spoken in Eastern Congo, Rwanda and Burundi.
From 1884 the Omani Arab Zanzibaris and Swahili Zanzibaris acting on the increased demand for mostly Ivory and slaves established a hinterland Sultanate that was answerable to Zanzibar based Sultan Seyyid Said the Sultan of both Oman and Zanzibar.
He encouraged Zanzibaris both Arabs and Swahilis to settle in Eastern Congo the source of their trade goods cutting off middlemen eg the Nyamwezi long distance traders and maximising on their profits.
Theese coastal traders were Swahili speakers including the Omanis who adopted Swahili as the lingua franca.
1884 a sultanate was established in Eastern Congo and Swahili became the lingua franca in this part of Congo .
The Omanis and Swahili would intermarry with the local congolese tribes and the process of Islamization went along with the process lf Swahilization....of what was referred to us "Uungwana" (civilized) and hence the dialect of the "Waungwana" being "Kiungwana" or the language of the free/Non-Slaves/Civilised population.
The capital of this hinterland sultanate in Congo was Stanleyville which today is known as KISANGANI.
Most people made no distinction between the Omani Swahili speaking Arabs from Zanzibar and the Waswahili from Zanzibar that was living in Congo at the time. So they were all called Arabs.
Waswahili were referred to as Arabs mainly by Europeans and sometimes both Omanis and Zanzibari were referred to as “Ngwana” by the local population. The “Baswahili/Muswahili” identity was introduced later on in Congo’s history.
The Arab-Swahili vs Beligian wars between 1886 to 1887 ended the Sultanate ....King Leopold II would control the so called Congo Free State as his personal property....
NOW Some homework from your history teacher.....which "Kiungwana" dialect words do you know?