Is this from the Arabic word for engineer, 'muhandis'?
Generally, if a word ends in "i", it comes from Arabic! If it ends in "a", it usually has Bantu roots.
That only holds true for verbs. Many Bantu nouns, adjectives and pronouns end in "i".
Shouldn't be An Engineer?
You are right but for this answer I found no way to report the error.
I answered "an engineer" and it is now accepted (31-Dec-2018)
Interesting. Is it semitic in origin? Like in Hebrew, מהנדס?
In fact, in this case both Hebrew and Swahili borrow from Arabic.
A engineer? Thats just wrong...
Yes. The word 'engineer' starts with a vowel, so it should be 'An engineer'.