It's similar to many Japanese words, which take English words and add vowels here and there.

It stems from English influence in East Africa, when they drove sinister.

In that case, I would say it's come from the Swahilization of "keep left" (keep-a-lahf(u)t-i ?).

Should I be correct, would it be improper of me to decline the sunk fence? I would accept a laugh, though.